Maybe it's partly that bass players often look for different things from their gear than guitarists - they tend to value clean headroom over valve-like overdrive.
Solid state amps are quite well suited to this, and with Class D amps becoming more available and accepted, it is possible to put a lot of power into a very small space. Obviously if you're going for light weight and compactness in a head, you want the speaker cabs to be as light as possible to match - there's not much point getting a 5kg head if your cabinet weighs 100kgs, so there is the pressure for manufacturers to make their cabs as light as possible.
Of course this is not true for all bassists, there are some who still want the sound of a (very heavy) all valve head...