I'm not sure I completely understand the discrepancy in the amount of love given to D20 Apocalypse and D20 Past. Don't get me wrong, I wrote a good chunk of Apocalypse, so I'm happy to be in the loved rather than hated column, but what do people find deficient about Past?
I've read 'em both, though I don't own them (I've found what I'm going to call the Gamer's Public Library locally, and it's much more efficient than buying for checking out books I will only need for the odd one-off or two).
d20 Past's biggest problem (and a problem I kind of have with d20 future) is that it's linked to hitorical reality. It's not about fantastic imagination, about weaving tales of adventure and exploration, it's abuot seeing how much research you can do and referencing as many historical figures as you can. And unless you're a big history buff, that's BORING.
This problem is, IMHO, directly linked to the concept of Progress Levels, which were in d20 Future, too. But in d20 Future, they served to anchor fantastic visions of the far-flung times. In d20 past, they more serve to constrain and confine what a campaign can achieve.
In d20 Apocalypse, meanwihle, PL's were kind of only obliquely referenced, and basically admitted that apocalypses will distort PL's because while the majority may be stone age, treasures may be far-flung future.
If d20 Past focused less on simulating WWII battles and more on, say, playing the group of heroes that infiltrates Hitler's secret compound where he's breeding genetic mutants and manages to kill him in a hail of gunfire, it would've been better. You get the impression with d20 Past that if you don't know the era in which you're setting the game, you gotta do some research! Which is dull. IMHO, it would've been better with "If all you know about the Victorian age was that men wore pouffy shirts, you can still play a rollicking good adventure in the Victorian Age. Even if you didn't know what Queen Victoria was queen of."
So it was kind of like the problem with Maztica. It focused too much on dry and uninspiring history and not enough on adventure, excitement, heroes, and villains.
Conversely, d20 Apocalypse pretty much works with the fantastic adventure and excitement. d20 Future ALMOST misses the mark, but manages to hit it more often than not.
Though the line is probably dead by now, I would've been interested with a d20 Ancients going into Greek, Rome, Stone Age, Mesoamerican, and other "primitive" tech levels. Dungeons and Dinosaurs, anyone?
Again, main problem with Past: no imagination. Apocalypse is kind of the opposite of that, where it leaves EVERYTHING to your imagination.