Location. Cost. Riding time.
Driving a long way. Then paying $10-$15 more than I can pay to ride outdoors during the season. And finally getting only like 3, 4 if I'm lucky, 10 minute riding sessions in all cause 2 or 3 people need catered and want a whole practice session to themselves. All of that has been the reason why I haven't rode indoors yet this winter.