Basically because it's way cheaper (supports have gotten better access to gold, but are still very starved for it) and generates wards while roaming (I've roamed as support for well over 5 minutes, which eventually SS will run out and I will have to back. Quite often, the items you can get by skipping SS help out a lot more than those wards (I skip SS on Zyra and it gives our lane so much more kill potential.)
Edit: That and there's a huge lack of variety for SS. There's only an HP option, which while tank supports are ok with it, other supports would rather get something such as mana, AP, or other stats that actually benefits them (Soraka doesn't really like it either.) If you get caught out as Zyra, Soraka, or Sona, 150 HP isn't really going to do you much good. I don't even bother with any defensive stats on Zyra (save Zhonyas) because I'll be dead regardless of defensive stats. There really isn't an issue with other roles building SS anyways considering how gold/slot inefficient it is and yellow trinket upgrade is just way better.