I heard a lot of people saying this, but I still don't get it. If Spell Vamp affects every single damage ability, why do I see it only on the ap carries?
Spell vamp is highly useful, but, excepting runes or champion-specific abilities (such as Morgana, Olaf, or Lee Sin), it is very hard to find on items.
The only item in the game with spell vamp is the Hextech Revolver and the items it builds: Hextech Gunblade and Will of the Ancients.
Hextech Gunblade is an expensive waste of itemization for any non-hybrid champions (like Akali, Jax, or Ezreal), and Will of the Ancients provides a straight AP bonus, which means it too is wasted itemization for non-AP champs.
In short, most AD champs that could benefit heavily from spell vamp (such as Urgot or Lee Sin), won't benefit from the AP that comes with it, meaning there are better items to spend gold on.
If there were pure AD + Spell Vamp itemization, you'd see spell vamp on a lot more champions.
Spell vamp only affect skills that deal magic damage. This includes most skills but there are numerous skills that do attack damage. Also not everyone need's spell vamp. To get anything from spell vamp you must first do quite a lot of damage. If you are an AD carry who for has a skill which does magic damage you'll be investing into AP instead of AD defeating the purpose of being an AD carry. Lifesteal would be a better effect for that AD carry.
Since spell vamp scales off the damage of your skills, usually the champions that do the most damage through skills are AP carries.
Also, all items that give spell vamp currently also give AP, which would be wasted on non-ap scaling heroes.
And finally, since AP carries generally are the ones that revolve around using their skills as their main source of damage, and spell vamp helps to keep them safe while doing so by providing a sort of offensive defense.
Sometimes, you can get spell vamp as support to help your team (Will of the ancients).
When a NON-ap carry get it, you lost one slot in your inventory. Even an ad carry can get spell vamp, Take a look here: Spell vamp / Life steal and AD scaling abilities
You have only 6 slots. You need the best set to your champions, and even Spell vamp being a good thing, there are others even better.