So... in summer it' so hot that the only game you can play is a relaxed game of baseball?
(besides an occassional soccer match)
So the reason the soccer games seem so "occasional" compared to the other sports is that soccer games usually happen on weekends, with occasional midweek fixtures thrown in here and there. Sometimes maybe a game on a monday, but it's rare. The reason for this is that the way soccer squads are put together and the amount of rest top players need pretty much means that if you want to play a midweek game every week (i.e. 2 games a week).. you gotta rotate all 23 of your players and rest them alternatively every game. This isn't great, because you want to see the same starters every game, ideally speaking. So what happens is most teams will play a game a week "or so". With some midweek games thrown in the mix out of necessity or for whatever reason. Believe it or not what you are looking at is a packed schedule. The most activity happens on weekends, which is why you're seeing the graph with spikes like that
Baseball on the other hand.. .. a team will play like 4 games a week. It's crazy. Hockey games happen all the time too, although less frequently. Basketball teams play a lot of games as well. Look at the top numbers so you can compare.
American Football is different, because from what I've been told players just can't handle all that violence. So you get like 16 games a season per team. Only 8 home games I believe.
I don't know why American football and basketball are winter sports here. I suppose in the case of basketball, all the teams play in arenas that are fully enclosed, with a roof and all. So you can play at any time of year. An odd choice to not give players the winter off instead of summer, but at least the arena setup makes sense. American football .. I have no idea why they don't play in the summer.
In the end this all works out well for MLS. It's a growing league and it would really suck to have to compete with so many other popular sports entertainment products. Competing with baseball only (for the most part) is easier because the demographics don't overlap as much. By that I mean.. Soccer in the U.S. is a sport that people in their 20s and 30s follow, whereas baseball tends to be a sport with older fans. But american football and basketball also attracts youth, a lot more than baseball does. So.. MLS competing with baseball in the summer is sort of ideal
There is also no way MLS could have a season that takes the summer off and runs through the winter. Toronto and Montreal, not to mention Chicago, Minnesota, and many other cities just couldn't support soccer games in the middle of the winter. It wouldn't work. We had huge problems in Toronto when our team had to compete in the Concacaf champions league in february 2 years ago. Our pitch got destroyed and the fans were freezing.