Yes, there is an explanation if you read a few pages on the starwars.wiki. The Grand Army of the Republic(GAR) was the main fighting force at the beginning of the clone wars, it was comprised entirely of Jengo Fett clones made by the kaminoans, which took about ten years to mature.
However, after years of war Palpatine turned to Spaarti cloning cylinders to complement the Kaminoan cloned army. From starwars.wikia.com:
"At the near end of the Clone Wars, all but a few were unaware that
Supreme Chancellor Palpatine had secretly commissioned a second clone
army, which differed from the Kamino-bred Fett clones in two primary
ways: they were grown to maturity in roughly one year as opposed to
ten years, and they were created on Centax-2 instead of Kamino."
This also shows why the stormtroopers we know and love from the original movies are so terrible at their job compared to the original clone troopers in the prequels.
"While the Spaarti Fett clones numbered in the billions, and were
thus possibly far more numerous than the depleted numbers of their
Kaminoan-bred Fett brethren, the Spaarti clones were given only
flash-training on the basics of being a soldier, such as how to fire a
rifle. When first sent into action, the clones of the 14th did not
bother to take cover from enemy fire, and their weapons accuracy was
surprisingly pitiful, as noted by the commandos of Omega Squad."
When the clone wars ended and the empire was formed, the original Fett clones were folded into the new Stormtrooper Corps, which also included the Sparti clones. However, though unwaverlingly loyal to the Emperor, Sparti clones had several problems beyond poor training, such as clone madness which could cause the clone to refuse orders and fire on their own troops.
In response, the empire moved away from clones and began to rely heavily on birth-borne human recruits to fill the ranks. With Kamino bred troopers being slow and expensive to create and Sparti clones being subpar soldiers, regular recruitment of human soldiers became the preferred method.
In addition, concerning the crews of Venator star destroyers and other ships, they would not consist entirely of clone troopers, but instead have a mix of clones, human officers, and assorted other beings to perform more menial tasks.
I hope this provides a viable answer to your question! Links to the star wars wiki where I got my info below! May the force be with you!
- Grand Army of the Republic
- Stormtrooper Corps (Legends)
- Spaarti Cloning Cylinder