Real good stuff.
We know three things happened that had nothing to do with steroids:
1) The game expanded twice, which meant a lot of iffy pitchers were suddenly throwing a lot of innings. Not coincidentally, 1998 was the second of those two expansions, and that's the point at which the homers really blasted off.
Even during the pitching heavy '60s, we saw power spikes every time there was a round of expansion. That's how Maris got 61. The difference between the '90s and '60s is that there was a greater depth of hitting talent in the '90s and a greater depth of pitching talent in the '60s. So overall scoring went in different directions during those eras.
2) Smaller parks came into vogue.
3) Hitters learned to hit the other way for power (thanks to growing up with aluminum bats). Suddenly you couldn't rob a guy of his power by pitching on the outer half of the plate. Mix that with a higher number of bad pitchers and smaller parks and you've got a recipe for a power explosion even without a single chemical enhancement.
And there's probably a lot of credence to the lively ball theory. We saw MLB experiment with the ball in the late 80s. In 1987 offense suddenly exploded like it was the 1930s all over again. Then, just as suddenly, in 1988 it was like the return of the Deadball Era. It split the difference after that, but it's entirely possible that after 1994 the league placed a bet on chicks digging the longball.
So a lion's share of the power spike very well could have been non-PED-related. After all, if pitchers and hitters were using roids in fairly even numbers, then you'd be looking at a bit of a cancelling out effect.