Because the left gear is used for wheel spinup.

I have about 20K hours in Boeings and made 9000+ landing in the 737.
The 737 NG engineering is awesome. Nothing wrong with the spoiler
system. Every SWA pliot expects spoiler deployment on touchdown. If it
does not occur, they immediately deploy manually.

If you make a good wet runway landing, spoiler deployment is never a
problem. "Plant" the aircrat on the 1000 ft. maker..

OBTW James, they are NOT "on-off mircroswitches" but proximity
switches. A lot more reliable than mechanical switches.