Crossovers, why so few options?

It seems like if you're in the market for an external crossover, there just aren't many options, particularly if you don't want a piece that belongs in a rack mount (e.g. Behringer, Rane).

Why is this the case? It seems like external crossovers would be in pretty widespread use, but the selection is much less than in the amp/preamp space. On all of Audiogon, there are only two or three crossovers for sale at the moment...