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Why is it that most professional tubist play piston tubas?

I didn’t.
Pros play for the sound, not the valves.

I have both piston and rotary on my two horns.
People have written about the differences:
https://www.normans.co.uk/blogs/blog/pi ... difference
https://www.trumpethub.com/rotary-valve ... -trumpets/
https://www.jpmusicalinstruments.com/ne ... ton-valves
https://www.yamaha.com/en/musical_instr ... sm004.html
etc., etc.

My own opinion?
I prefer pistons simply because that is what I was brought up with. I can do most valve maintenance myself.
If I was playing something slow, low, or melodic I would choose rotaries.
But, if faced with a presto passage with lots of notes, I would go for pistons.
As windshieldbug says, sound has top billing, though.

Dreaming, and if I had limitless funds, I would remove the mechanical linkage of a rotary valved tuba and replace it with micro-servos.
The result would be very interesting.....

It has been done with limited success. https://urish.medium.com/designing-3d-p ... 38dcb65b39
I guess the fun of playing is mastering the skill needed to get the result you want.
The logical conclusion of the argument is to put a CD on and enjoy someone else doing it better!
This attempt at t'bone amused me. https://blog.arduino.cc/2020/12/29/the- ... -trombone/