You can make pick up games of all sports out of trash. Difference is, you can play soccer that more closely resembles the real thing far more cheaply than you can play baseball. Find an open field, a ratty hand me down ball, some trashcans to mark off goals, and you have a decent representation of the sport. It can also be played in a variety of different sized spaces (think indoor or street soccer). You need a large space to play baseball. Baseball on a basketball court sized area would be in' stupid. No kid wants to just hit a ball 90 feet. You need decent distance between bases, too short, and you'd never throw anyone out. Balls are a bit harder to come by because make shift balls out of wadded tape, paper, whatever get destroyed practically as soon as they're hit. Tennis balls also don't last long. You want something hard and durable, and then you need gloves.
Soccer is just way more accessible. I live in a pretty wide open suburban neighborhood, with a 150 x 75 foot field next door. That's a decent size space not many people have immediate access too and it's still too small and narrow to really play decent baseball, especially given the fact there's a building on the left and houses and parked cars on the right, so hardball is out of the question. Only baseball you can really play is homerun derby with whiffle balls and every time I take my friend's kid out there to hit, we lose balls to the roof of the building. But it's a perfect area for 5 on 5 soccer.