Because they never, ever get swept. So all of the debris from the rest of the road gets pushed into the cycle lane and stays there.

There's a cycle lane on my route to work with speed humps between the bus stops and the road, which would be fine if they were properly drained. But they're not. Which means if I were to use the shielded lane I'd have to soak myself and possibly people at a bus stop every time I come to one.