Because they are utterly pointless on the street. They shave hundredths of seconds off your lap time on the track, but you will never notice any improvement when using them on the street. Aerospokes will give you a rougher and nosier ride, slower acceleration, and increased stopping difference. But of course, anyone who uses them on the street hasn't ridden a bike long enough to understand such things.

Why do people ride aerospokes on the street? For looks, and nothing else. I have a set of old Specialized carbon tri-spokes on my century bike (an old Marin triatholon bike). My century route has no stops or pauses along it's entire distance, and I ride for time, so they aren't a bad choice. No acceleration, no braking, just steady speed over a very long distance. In such a situation they are a good choice. The rear tri spoke is threaded on both sides, so it can be used with a fixed cog or a freewheel, so I could easily use them on my fixie, but I'm not "cool" enough to bother.