Support wouldn't be such a rare role if it was more fun/rewarding.

Being constantly under leveled, under farmed, and under itemized leaves support a very crappy role to play as.

Several support champions get nerfed simply because they become useful in other lanes (such as mid) leaving their damage consistently on the lower end of the spectrum. While few, this does admittedly have exceptions (Morgana, Zyra, and Karma come to mind) But the over all case in point is that the role is... boring. You are expected to support another player who may or may not be good, and it is exceptionally hard to carry as a support. It's not impossible, but it is harder than probably any other role in the game at it, and there are certain support picks who just can't, meaning their are some better than others.

Support items have been coming back, which is cool, but you can't buy them if you don't have any gold for them. The first buy support items generally don't give back as much gold as simply farming could, especially with the runic shield. Not to mention the stats on them are kept deliberately weak so they aren't bought in other lanes.

There is a pattern here.