I'll have to look and see if they called them Fixed Points, but we've seen them before, in the older episodes; I just don't know if they were explored as being such. There have always been things that the Doctor could change... and things he couldn't; most often, the later were ones that would interfere with his own personal timeline, and thus generate a powerful paradox. He was never very good about explaining WHY, tho.
Some examples that leap to mind include various character deaths (Adric, for example), the Great Fire of London (said to be going to happen regardless of their actions), and the destruction of Ockora in the Second Doctor's day.
That being said, I think one of the reasons they weren't referred to as such relates to the fact that the Time Lords had considerable ability to manipulate / control such things. With the Time Lords in existence, there were rules (The Laws of Time, specifically) about changing such things, but it was possible, with some unknown support from the Time Lords.. With them gone, it looks like the natural laws have a tighter hold.
In Father's Day, the Doctor mentions that the level of paradox Rose caused could have been dealt with when the Time Lords were still around ("There used to be laws preventing this sort of thing"), instead of the Reapers coming to sterilize things. It's also mentioned, during the Rose arcs, that the walls between alternate universes were more permeable, as well. This suggests that SOMETHING the Time Lords did, or some aspect of their existence had a fundamental effect on what otherwise would be natural laws.
So, I think Fixed Points existed.. But didn't used to be quite so powerfully locked down. The absence of the Time Lords has caused them to become the more significant things they are now.
This is all MHO, tho; I've yet to see anything clearly explaining it. YMMV.