Out of all the supports, why do people not want to play Nunu support?

[P] every 5 basic attacks gives a free spell (includes ult)

[Q] Nunu heal, wave clear, large minions are easier to kill for ADC (other possibilities)

[W] long lasting AS and MS. can recast as soon as or not long after it has ended.

[E] damaging, MS and AS slow

[R] MS and AS slow, and major damage to those in it.

I've been able to use his kit numerous times to weaken opponents to certain points for my ADC to get easy kills. I love his kit. Why then, do I hardly see anyone want him as a support? I went into a Team Builder as Nunu support and Twitch was cautious and just left causing me to be kicked. (The next group took me without question and we destroyed bot. Also almost level 4 mastery with Nunu.) I've played every support and out of all of them, Nunu has the most straight forward kit I've seen.

I want to post this in hopes of seeing more Nunu's in the support position. As great as a jungler he may be (we all know it) he's more fun in the support role (at least thats just my opinion, don't take it harshly please...). Just asking for more diversity in the support role since we finally have 12 supports and Leona (no one will admit she's a primary support rather than tank).