The actual mining is done by clients using software like cgminer; the pools just coordinate. If the software doesn't support a particular POW algorithm then it would have to be updated and tens of thousands of people would have to install the update. Also, there are pools that auto-switch between different scrypt currencies to mine the most profitable one, but mining software doesn't (currently) auto-switch algorithms.
The actual mining is done by clients using software like cgminer; the pools just coordinate. If the software doesn't support a particular POW algorithm then it would have to be updated and tens of thousands of people would have to install the update. Also, there are pools that auto-switch between different scrypt currencies to mine the most profitable one, but mining software doesn't (currently) auto-switch algorithms.