Yeah, almost certainly just a practical choice, for whatever reason. Maybe a few of all the other amps coming out of that factory already runs those 6L6 tubes, and they cut down on the purchase list. Lots of economy of scale at play.
Back in the 90’s, I direct ordered a purple SLO 100 with KT66’s. They were offering then at the time. From what I recall, that was the sweetest SLO I ever owned, and I owned many.
How does that make you feel? Some people go crazy about tubes. But the reality is, if you can't make a good sounding amp with what is readily available, you can't make a good sounding amp.