Because Warmog's is bad on Soraka, a support caster who stays on the backline and would rather have AP, CDR, and utility over tankiness.
She also can't afford it as a support, and mid/top lane Soraka got pretty gimped by the loss of self heal. She has a weak early game, if you're going for something like Rylai's or Visage first, and doesn't have anyone to heal in a solo lane, depriving her of a lot of her power.
Also, her W heal costs %max health. So even if you're healing a lot, your heals cost even more with higher max health. Having more health hurts your value from flat health regen and potions. You also heal less per heal than an AP build, so you have to heal more often, costing more health.
You also won't finish your build for a very, very long time, even in a solo lane. Warmog's+Visage+Rylai's is 2750+2700+3000=8450g, plus you'll want boots. Pretty much impossible for a support who needs Sightstone/wards, and by the time you have that, your opponent has
, or
and can still kill you.