Why creature enchantments aren't playable?
The simple, no-frills answer: creature enchantments aren't generally playable because they offer your opponent an easy 2-for-1 (or better, if you're the sort of person who likes to load creatures up with multiple Auras...)

Any removal spell that would have previously just taken out your creature now takes out all enchantments on that creature too, giving your opponent an easy route to significant card advantage.  Not a situation you want to enter into lightly.

EDIT: The OP asked in the comments about Totem Armor.  See this article by Mark Rosewater, in which he says:


  The idea behind totem armor was to offset the inherent card
  disadvantage of Auras. Normally when you play an Aura on a creature
  you have the vulnerability of being "two-for-oned"—that is, your
  opponent can spend one card (a kill spell) to make you lose two cards
  (your creature and your Aura). Totem armor prevents the two-for-one
  because it saves your creature. The Aura acts as a safety net.


Straight from the horse's mouth there!