Steroids alone didn't. Smaller parks, more teams, tighter strike zones, tighter wound balls, shift in way game was played. You can reasonably argue that greenies affected games daily more than steroids. Plus it wasn't just hitters on steroids like the media wants to present.
respectfully but completely disagree
steroids made some good players into MVPs (like Ken Caminiti), or guys who sets new standards (40/40 club of Canseco) or made great players like Bonds otherworldly. They changed the ways that pitchers had to approach guys like Bonds and McGwire because there was no safe place in teh strike zone to throw them a pitch - accordingly their batting averages and walk totals as well as their power numbers skyrocketed. They restored Clemens from a washed up pitcher to a guy who absolutely dominated in his early 40s
you can talk about ballparks, balls being wound tight, special bats etc - but IMO all that is just rationalization with the intent of avoiding the obvious. There are tons of ways to cheat - but nothing comes close to what steroids/PEDs did to change performance in baseball