Because anyone who plays support knows that all those double kills in the bot lane is the supports doing, not the AD carry. All the AD carry has to do is right click and occasionally press Q. The support is the one doing all the work. Since these days few people play support, most people don't know that so when some shit goes down at bot lane and the AD carry walks away with a tripple, everyone just assumes the AD carry is good. No friends, the support is good not the AD carry.
I main neither ADC nor support but I have played both roles extensively and I'm telling you, I have a far greater impact playing as support than as ADC. Whenever my bot lane loses, I blame myself, not the AD carry because all the AD carry has is damage. I'm the one that has the shields, speed boosts, the cc, the heals etc. I have more tools than the AD carry. He just has multiple ways of applying damage.
My go-to support is Leona and there are days where I can make even the most braindead ADC players look like Gods simply by out-playing the enemy bot lane.
I'm seriously considering maining support or more specifically Leona/Naut in Season 7. I don't think anyone, especially at my elo(Silver) has any clue just how powerful the support role really is.