I guess there are a few aspects that influence the transformation of dead bodies to necromorphs.
To start of, we have to look at the ways, how necromorph life is created:
The Marker signals, emitted at different wavelengths seem to have a kind of targeted, mutagenic effect on biomass. These signals, which could also be translated to a genetic code, alter the genome of a cell and results in a microbial-like, self replicant life (it seems also to transform mammalian tissue to this microbial, recombinant life-form). This new life-form gets active once it comes into contact with dead tissue (like epidermal cells) and seems to use this a nutrient, which indicates that this life-form is a kind of necrophage. A very high mutation and recombination rate was also observerd by the researches working on the Black marker.
We ran into a disturbing, but fascinating find. In re-creating the DNA instructions gleaned from the Black Marker, we managed to create a recombinant microbial life form. It seemed dormant at first, even when placed in a bath of rich nutrients. Was it not for Dr. Clifton's poor hygiene, we might have deemed the attempt a failure. Fortunately for us, he had not fully decontaminated himself before working with the sample and managed to contaminate it with some dead skin cells. Suddenly, the microbes jumped into action! Recombination happened almost instantly and the sample began self-replicating and mutating before our eyes.
- The Red Marker - Background Log from Dead Space 1
The Corruption consists of this microbial life-form and is a macroscopic, strong smelling habitat-changer. This is the growing flesh, which Isaac observes around the Ishimura. Its purpose could be to spread the genetic information in the necromorph life-form. One guess would be, that this is one form, how the infestation is spread. Even if the Corruption is growing very fast, the infection of human bodies would last longer, because it has to come into contact with them and also has to deal with a higher complexity of cell-cell-communication in order to create the necromorphs.
Considering a evolutionary mechanism from the "real world" called Horizontal Gene Transfer, meaning the ability of bacteria and other microbes, to exchange DNA between each other (In comparison: Vertical Gene Transfer means the transmission of DNA from one generation to the next (Reproduction)), this could be a way, how the necromorph cells infect the tissue with its genetic code to induce the transformation. That the prey has to be dead to enable the infection, could be, because the immune system would be silenced at this point to avoid any resistance to the transformation.
As @Clockwork mentioned, the Corruption is also aiding the killing of the prey by releasing toxins in the air and slowing it down (because of its sticky surface), allowing the necromorph to kill it more easily. This would be a positive feedback regulation, as the provision with dead bodies would also benefit the growth of the corruption and the total amount of necromorphs, which could then aid the provision of dead bodies.
No doubt you're aware of the organic growth found in some parts of the ship. I've been examining the substance and thought the results would interest you.
The cells most clearly resemble bacteria, with viral behavior. Like bacteria, the organism infects other cells through osmosis, then mutates and reproduces agamogenetically.
What's amazing is the SPEED at which it grows. A single bacterium reproduces twenty-fold in just one minute. I've never seen anything like it. I won't stake my reputation on it, but I think this may be a genuine alien lifeform!
Prof. Hawley theorizes that it's a habitat transformer -- the levels of gaseous output are tremendous, which is why it smells so bad -- but to what end we simply don't know.
- Corruption (Cross`Log) - Text Log from Dead Space 1 Chapter 6
The third way of creating the necromorph life-form is by direct infection by an Infector or the necromorph pathogen (as Dr. Mercer did in order to create the Hunter in DS1). This is the fastest way of how necromorphs are created, but the dead body has to be penetrated by the Infector in order to inject the pathogen/tissue into the body to induce the transformation. An Infector is always the starter of an Infestation (as seen in DS Downfall and also reported in a background log). The Infector is created by the more slowly way (meaning the contact with the microbial life-form created by the marker signals). It is mentioned, that the total transformation took about 3 days (which is much longer than the amount of time an Infector needs to transform a dead body).
Day three of quarantine for the former Dr. Clifton and Hogan. The infection was spreading quickly through Dr. Clifton. Dr. Hogan appeared unaffected at this time.
The chest of the corpse started to expand like it was taking a deep breath. Something appeared to be happening under the surface of the skin. The body was convulsing. A yellowish briny liquid bursted from the mouth accompanied by choking noises. There was a crunching noise as the throat bulged and the jaw collapsed. The head was splitting apart, reforming now into what appeared to be a proboscis and feelers. […]
- Recombination Study - Background Log from Dead Space 1
To sum this up: There are two way, how the infestation spreads. The first one is the slower one, which seems to focus on changing the whole environment and creating the Infectors. The second way are the Infectors, which are able to create Necromorphs much faster. Together it seems, that a typical Necromorph infestation works in a exponential way, with an early lag-Phase and getting faster, the more Infectors are created.
So, why are not all bodies on the Ishimura already transformed to Necromorphs. Considering the two way, I would guess, there are two main arguments.
The first one is related to Marker 3A, the red Marker. The Corruption and the necromorph life-form seems to be directly related to marker signal (in order to provide the genetic code to create it). As we know, the red Marker is man-made and also very small. So I would guess, the marker signal is not strong enough to allow the Corruption to spread fast enough, to infest all corners of the ship in the given time (Especially if we compare it with the much larger, golden Marker from DS2, which was able to induce the infestation of the whole Sprawl in much less time).
The second argument is related with the time, at which the game takes place. As we know from DS Extraction and DS Downfall, the USG Kellion arrives not that late at the USG Ishimura (Lexines warning at the end of DS Extraction which was sent to the Kellion, which is just arriving in the orbit of Aegis VII). By this, I would guess, that there was simply not enough time for the marker to create enough Infectors, which would then be able to transform all dead bodies. This would also come in line with the observation, that Isaac encounters more and more Infectors the longer he stays on the ship and the more time passes, allowing them to be created by the marker.