Why Supports Do What They Do & Why There Aren't Many Supports (Long Read)
Now, I just wanna say as a disclaimer before I dive into my explanation; this is my personal opinion. It's what I've learned and lived by as a support main. It's what I explain to my ADC when they ask the "why" questions that always come after a good or bad decision. I'm not trying to make the ADCs look bad, nor am I justifying when I do make bad decisions. It's just to get people on a generally same-page basis as to why one of the rarest roles to fill does what they do, and why there are so few people who play it to begin with. If I think of more things that I can explain, or if someone comments an idea I can relate with, I'll add them here.
Why Supports Do What They Do:
1. If you do something incredibly stupid, I'm not going to follow you. If you go tower-diving for a kill, I'm not going to follow you. Get the kill or not, I can't save you from a tower that does increasingly more damage, and I know it from past experience. I can wait on the edge toward our side and try to save you if I think you can make it, but I'm not tanking a turret for you. I'm a support, not a tank (other than in rare cases like Leona or Alistar, but they don't have large heals or strong shields to give the ADC that can really save them if need be).
2. If you're not in lane, I'm going to take CS If you die, or if you're backing, I'm going to farm minions while you're gone. You can't expect me to leave a whole horde of 30 minions for you to farm when you get back. If I don't get them, our minions or the turret will, and that's just sending free XP floating out into thin air. I could use that XP and gold to buy items, level up, and make use of myself.
3. I'm going to be where I need to be, not where you want me. So many times I've had the team ask me why I wasn't in a team fight, then shortly after I get to the next team fight, ask why I wasn't saving the ADC who was pushing lane and just died from a gank. The long and short of this is: I can't be everywhere. I can't be warding, and helping the team in a team fight, and helping the ADC push. If it's more important that we keep minions away from the exposed inhibitor while the other four members are in mid team fighting and I'm at the shop, I'm going to save the inhibitor then come help with team fight because it's on my way. If you're face-checking dragon and I'm still walking down the lane, I'm not going to go straight to you because you're either going to run into them taking dragon, or it's going to be clear and you can ward it.
4. Let me do the dangerous stuff. Your job as ADC, or whoever I'm following around in team fight, is to get kills and get fed. If we think they're at Baron/Dragon/Buff, let me go check it out. If they're there, you're free to follow. But since I have the wards, I do the face-checking. One kill on a support is better than them getting a double because you followed too early, or a triple because we all went as a group with no vision. One kill is always better than 2 or 3 for the enemy team, and if I did it right, I died to place a ward, so it was well worth it.
Why There Aren't Many Supports This is one I actually didn't know myself up until becoming a frequent support player, and it is in part just a theory. Each part could be wrong, or it could be spot-on. But it's what I've found to be the reasons why many people refuse to play support.
1. The job isn't easy. Next to jungling, supporting is one of the most difficult roles to play if you're used to being in-lane. Your job isn't just "go here, farm, kill champion, farm more". No. Your job is to ward as much of the map as you can with only 3 wards at a time in a way that gives the most vision, save your ADC if needed, follow the team in a team fight and save them if needed, face-check so your ADC can farm safely if there isn't a ward down, sacrifice yourself for your team if needed, don't take kills unless you have to, or secure a kill if your teammate can't, and much more.
2. It's a big change from being in-lane. Top, Mid, and ADC all have the urge to sit back and farm. With support, you can't really do that. Most ADCs will tell you to keep your mitts off their minions. You have to be everywhere at once, much like the jungler. However, your main job is getting someone else fed, which makes you heavily relied upon. As jungler, you don't HAVE to get a kill or give a kill if you gank. It's not always what's expected. Just pushing the opponent out of the lane gives opportunity for farm, and sometimes that's all you need.
3. You carry a lot of the blame. If the ADC is bad at what they're doing, you still carry the blame as a support in most cases. ADC died? You didn't save them. ADC didn't get a kill? You didn't help them get it. ADC didn't get farm? You must have been hitting minions and messing up their last-hits. Team didn't all make it out in a team fight? You should have saved them. Enemy took dragon/baron? You should have used more wards. The list goes on and on about the things that can be pinned on you that no one else can be proven to hold the blame for. The outcome of the game heavily relies on how well you do, and it very well should. But that just makes it twice as rough when the team loses.