It's been a long time since Solaris worked well on a 32-bit CPU. UltraSPARC replaced SPARC in the 90s, and Solaris on x86 was not a very common thing then. It wasn't until Solaris 10/OpenSolaris (around 2005) that the x86 port was really viable, and then 64-bit CPUs were really recommended (especially after ZFS came out in Update 1)

Even so, the syscall handler would have been written early in the development of the x86 port, well before it was "viable" for customers. Relying on quad-word instructions in hand-written assembly for such a crucial part of the OS would have meant no 32-bit support at all. That's probably not a good product decision even today, but at the time, it would have been crazy.

If there is going to be a debate over how mov performs versus push/pop, someone should just go ahead and change it. The time talking about it would be better spent on other things.