By 1993, those 64 pitchers would have accounted for 45% of the starting pitchers in the major leagues; due to expansion and the adoption of the five-man rotation, major league hitters were now facing, theoretically, the 140 best pitchers on the planet. In 1998, that number went up to 150.
That means that when balls started leaving the park in ever-increasing numbers in the mid-90s, batters were facing 86 pitchers who wouldn't have been able to get out of Triple-A for more than a cup of coffee 30 years before. Can you wrap your head around this? A player exactly as good as Hank Aaron couldn't help but surpass him in home runs; a player not even close to as good as Aaron has a chance.
This is the thing you have to understand. The more you expand a league, the more striking the excellence of the best players will be in comparison to those who have come before. If Hank Aaron is a 100, and Joe Shlabotnik is a 95, but Aaron played in an era where 16 teams used a four-man rotation and Shlabotnik plays in an era where 30 teams use a five-man... the relative difficulty of the league is vastly different for the two players, because the average player in the later era is not nearly as close to competitive with that era's stars as the average player was in Aaron's era. If Aaron's twice the player an average player of his time is, Shlabotnik will be three times the player an average player in his era is.
Now, one could argue that the reverse is also true; Warren Spahn pitched in an era where only sixteen teams worth of major-league caliber hitters could face him. Unfortunately... that doesn't work out the same. Teams in that era only carried 8 or 9 pitchers, generally; sixteen teams with 16-17 hitters comes out to 264 hitters in the big leagues. Now? 30 teams carrying 12 pitchers almost uniformly; 390 hitters. The ratio is not in favor of the hurlers here.
And, really, the excellent pitchers? Their performance is in line with the changing ratios. In the seventies, you'd see pitchers have insane seasons we don't see now, but those are more a result of usage patterns than anything else (and it's certainly of note that the last truly insane season by a pitcher -- Steve Carlton's 27-win season -- occurred prior to free agency). However, pitchers would fluctuate from godly to mortal on a regular basis. Go look at how many bad seasons the likes of Catfish Hunter, Steve Carlton, Vida Blue, and Jim Palmer had. Now? I bet you can name a dozen pitchers who seemingly never have a bad season off the top of your head, at least until they're walking with a cane. Their "bad" seasons are "bad" in comparison with their normal performance, or are explainable by injury. Trust me, you will find about one pitcher in the 70s-80s era who was a consistently awesome as a Maddux or Glavine or Clemens or even a Schilling, and the dumbasses with ballots won't even put him in the Hall of Fame.
Lastly, the really good pitchers in Aaron's era? They were still in the damned game in the eighth inning on a regular basis. Nowadays, they'll yank a guy after six because he's thrown 101 pitches, and replace him with a pitcher who is not as effective, even with the starter tired. (There was one season where Pedro, even after 100 pitches, was still a better pitcher up to pitch 130 than anyone in the bullpen.)
In short: I cast aside the notion that performance-enhancing drugs explain the increase in offense. It's all about expansion, theories on pitching to contact (hence the disappearance of the 300-K season), and changes in pitcher usage. I'm not saying Bonds isn't juiced; I'm saying that everyone who thinks steroids make any difference at all in the greater scheme of things is simply blinding themselves to reality.