The easy answer is that the hobby wasn't looking for them. Called them the uglies or diatoms and moved on.
But that answer is too easy, I think.
The other answers are wildly speculative, controversial, but I think some might be true.
Widespread use of GFO, ultra sensitive PO4 tests, and LED lights with more selective wavelengths than the T5/MH more common in the past... all make "normal" algae easier to control, without making a dent in dinos.
One day we ought to do some hobby archaeology and dig up old pictures of nuisance "algae" from before the microscope identification trend, and see how many we would retroactively declare to be dinos.
See if the prevalence has really increased as much as the long time hobbyists suggest it has, or if this is entirely a bias of more careful diagnosis.
The only two algaes I ever remember 12 years ago was a bought with cyano and green hair algae, that is it. I