First off, it really isn't artificial gravity, it is radial acceleration (often known as centripetal force) - AlanSE's answerAlanSE's answer on the 'Size and rotation of a station' question describes how radial acceleration is very different to gravity.
The major reason this is unlikely for some time is that in order for radial acceleration to feel like gravity, you need a very large diameter. Over 200 metres according to his calculations. This presents a couple of major problems:
- Getting all the materials to orbit. A 200m diameter station or spacecraft will weigh a lot!
- Building it securely so punctures from space debris don't cause major problems
- Building a stationary (or contrarotating) hub for comms antenna/docking etc will require spinning seals...complex and likely to fail
Basically, we could do it - it would just be incredibly expensive and dangerous.