Why is there no instant replay in football?

Just last week, in the Chelsea-Manchester game, Didier Drogba was kicked in the chest and flattened at the edge of the box. The ref gave him a yellow card, presumably for acting or wasting time. The replay made it obvious that he was viciously fouled.

That's only one of many instances of bad refereeing I have seen that could have been rectified with instant replay. It's past time UEFA/FIFA/etc. came into the 21st century.

Cheers,
Michael