Couple of things with the ESPN Radio / MLB relationship...
First, to be able to carry the world series you have to commit to their entire MLB regular season package which means normally a Sunday evening game of the week and picking up an additional Saturday game of the week starting in early August or so. This can be problematic, as others have said, if a station is already committed to carrying an NFL team (which normally plays on Sundays), a college football team (which will certainly have conflicts with the Saturday games), or any other baseball team (which will be playing Saturdays and Sundays). They also require that you carry the double headers on most of the major holidays (July 4, Labor Day, etc) and the all-star game and home run derby. There is some leeway in the amounts of the regular season that you have to commit to, but it's still in the 80 - 85% must carry area.
For the post season, they require that you carry ALL of it. Not just the WS, ALCS, and NLCS but also ALL of the Division series games and possibly some additional pennant race implication games.
On the technical side, this year is the first year that ESPN has used a new type of satellite receiver for the MLB games so if stations wanted to carry it this year they would have had to install and wire up a brand new receiver. This means new audio wiring, new automation wiring (if used), and this new receiver also has to be hooked up to the Internet so it can talk back to ABC Radio Networks. For stations that may not have a full time engineer, this could have been a factor.
And finally, as always, there's the sales side of things. If the sponsors aren't there, chances are the station isn't going to go to the trouble of getting all of that other stuff set up.