Support main here:
The most frequent reason I find myself NOT wanting to support isn't because of the Sightstone. While it's not truly gold efficient anymore, thanks to the devaluation of Sight Wards, Vision in and of itself is HIGHLY valuable.
If anything were the one true reason, it'd end up having to be: Focus.
I'm not High ELO. My MMR's in the shitter, and I accept that I'm /bad/. Therefore, I preface my following statements with this caveat: I am Low ELO. My Opinion is that and nothing more. Anything that I say is subjective to my personal perspective, and therefore, can and likely will be flawed, when compared to Plat+ Support Mains.
Focus is the bane of the support. In and of itself the word can refer to many things. Focusing objectives, something I'm prepared to do, but I find my team less so, instead focusing on CS/Jungle Camps, so they can get their last 30 gold for an item. Turrets, Dragons, and Baron control are vastly more important to me.
Focus on vision control. Even when I buy sightstone, that's only 3 wards I can place at one time. The Tier-one oracles replacement is fairly weak and situational now, good for setting up ganks, and clearing objectives of vision. I can also have a control ward. That gives me 4 vision options to work with. Now, Farsight Alteration is free, and has no limit. Yet, when we hit Nine minutes, I barely ever see them. Instead, I'm expected to Ward Dragon, Herald, 5 river bushes, 2 tribushes, and anywhere deep in the enemy jungle I can get to. Season 3? I could totally manage that. Back then, however...most of a support's power budget and gold budget was DEVOTED to vision.
Focus on setting the lane behind.
Dear enemy team,
I get that Bottom Turret's weaker in Defenses than the other two. I also get that it's an amazing objective, that gives you free reign over the dragon, while setting our AD Carry behind. It sets me behind too, and while I may take out one or two of you, devoting a Teleport, a roam, and a gank, all at once, to the sheer purpose of Eradicating me, the support with no kills and maybe 2 assists (if my laner's been lucky), isn't all that gold efficient. Either get the kill quick, or just threaten me until I have to back away. Don't chase me, don't flash mastery icons at me, that's rude. I get it, your 3/0 level 7 Lee Sin was able to QRQ me. It's very skillful of him, he popped the Karma. Very impressed.