There are probably a lot of reasons for this. I suspect one major reason has to do with the fact that archaelogy started in, and was focused on, the roots of western civilization, which pretty much means it took place in the neighborhood of the Mediterranean.
I think that's true to some extent. Also, don't underestimate the impact of politics. From the beginning of WWII until the end of the Cold War, approximtely 50 years, technology like satellites and underwater gear were being developed that makes these new theories possible. Yet during the same time vast areas of the globe were out-of-bounds for archaeological research.
Simultaneously, the politically correct 'out-of-Africa' theory was being hatched. Most tenured American archaeologists were blindered by the 'religion' of certain prevailing belief models that prevented them from considering any evidence that went counter to their credo. I don't personally presume to say whether that credo is right or wrong, but real evidence has a way of not going away (unless it can be be destroyed for political reasons as in the attempts on the remains of Kennewick Man). A whole lot of interesting evidence is beginning to pile up, and if the archaeologists in the old guard don't deal with it, a new generation will.
On the other hand, if a civilization were developed during the last inter-glacial period, it probably grew up in the "best spots" and was simply covered over during the following glaciation.
Archaeologists would have found both types of civilization generally inaccessible until just about now.
I am not holding my breath waiting on someone to find a flying saucer or transistor radio, but some stacked stones would seem to be a feasible discovery.