Good question. I don't have a clue what the answer is - but that has never stopped me from going on at length. /w3tcompact/icons/smile.gif
High-zoot hydraulics are in nearly all the really big stuff - Caterpillar and Komatsu have computerized "HST" that would knock your socks off. The biggest off-road trucks have been automatic forever. Many industrial TLB's are all hydraulic. Size, power and torque are not an obstacle. But cost is.
I think the mid-size utility tractor market just isn't quite ready to pay for this feature. Many ag tractors do a lot of steady-state constant-speed work which doesn't benefit from hydraulic drive.
Markets have inertia - they are slow to adjust - another good example is tire size - why are the rears so much bigger than the fronts if the fronts do most of the work? Because that's the way we like 'em, always have, always will, don't fix it if works, etc. etc. In what almost seemed like a practical joke, New Holland put the loader on the back - made so much sense the market should have snapped it up right away - but it's catching on very slowly. One dealer I talked to said people come and look at it, agree it's a great idea, and walk right out again. Sorry, the topic is drifting. Trolling for Mark Chalkley on the unequal tire size thing.
IMHO, HST is coming to Kubota's bigger rigs - just not real soon. My guess is that first there will be an HST L56 or L60, something for all the L owners to move to if they want more than the
L48. Too bad there aren't some european tractor magazines like the car ones - we could all be looking at sneak photos of the new Grand L 5210HST or the Anniversary Edition All-Dark-Grey M6100HST Vinyard Monster. Drifting again. Hey - How about them Patriots !