Why is Karma not played as much as other supports?

Solution 1:

Karma is just not very suitable support, she needs farm and items to be effective. That's why most people who play her play AP carry karma mid or top.

Her early game is huge with shield and healing wave(which heals and deals dmg) and this is the build i found most effective: Start with boots or dorans ring(if you dont expect early ganks) and max shield first(dont take point in your chain skill which is E) and max Q 2nd and take E skill only when you must(i think lv15). Build your WotA(get revolver at 1st buy) and couple of dorans rings. Proceed to rabadons deathcap and rylais crystal scepter. For boots buy ionian boots of lucidity or mercury treads(since that +20 Mpen in not very needed). Other items are situational and are something like void staff, abyssal scepter etc.

For laning i suggest walking to an enemy and activating mantra, then your shield followed by q and repeat it each time you get 2nd mantra charge. If you are unable to come close to an enemy activate mantra and shield a minion near the enemy champion. Teamfighting is basically the same thing but you may want to heal your allies a few times in teamfight(well placed heavenly wave can heal all your team).

Hope this helps!

Solution 2:

You can try this guide for reference http://www.mobafire.com/league-of-legends/build/the-comprehensive-karma-72191 This guide seems to be liked by many people. And for your previous question I've heard something about Karma not actually being that helpful as a support and would be better as an AP. I can't really give a judgement on this since I do not play this champion. I hope this helped you some.