Is it because of the tone?
It is partly that.
On gigs I often use ampegs as they are the house amps.
They are sometimes nice but to me usually feel real slow and not as clear,open and responsive as I'd like.
Also, conventional bass-amps with tubes are in my experience often noisier than SS ones and therefore not ideal for some of the kind of recordings I do involving acoustic instruments and room mics. I had the chance of trying a Mcintosh power-amp on a recording once.
That worked well. I still preferred my own power-amp though.
And there is the fact that not all tubes sound the same and one I might like may just not be available when one breaks.
So it is a maintenance issue as well.