I can't carry games from the role. This is going to sound pretty arrogant, but I play with generally lower win total / lower win ratio r/l and longtime gaming friends. It turns out that if I'm not carrying games we lose more often than if I am. Support isn't going to win the game for their team.
Support matters. If I'm significantly better than the enemy support I will get my carry fed. I will manage vision better on the map, and I will set up better engages as much as I can. I'll keep track of spawn timers, and summoner cooldowns. (the latter being a skill I'm just recently adding to my repertoire) There's a lot of good and helpful **** I can do.
I can have an absolutely huge impact on the game as a support. I cannot, however, win it outright. Sometimes leadership and coordinating is enough. Other times I get the "Don't tell me how to play!" guys. If the ducks don't line up in a row, so to speak, it doesn't matter how good I am as support, the team can effectively throw the game without me being able to stop them.
I like to win, and I've found my best chances are not always in the support role.
The other reason is because I actually pay my dues in regularly doing everything my team needs but jungle. I will even jungle if I'm absolutely forced to do so, and it's passable but it's definitely my weakest role. Once I've supported several games in a row I don't care if someone's never supported before, I need to do something different.