In order to have a stereo chorus the amp needs to have 2 separate power sections. A stereo tube amp would be heavy and not very cost effective to build. There are rack mounted stereo tube power amps but I've never seen a stereo tube head or combo.
I think tube guys like all-tube purity. Analog chorus uses a BBD, which would be impossible to implement with tubes without an amp the size of ENIAC. Digital chorus requires A/D/A, which even harder with tubes
What was that amp that Harvey designed that he was just talking about? He had a tape loop in the thing, that could possibly have done it and maintained tubish purity