simple, because noisy, Petrol station station no noisy just car/bus/lory in and out.
LRT  will have 70+ db noisy sound went rail pass it, although can install sound insulation, but still have small noisy,
and close to house will get may objection from residency, than will involved  politician will easy become politic issue.
you may see example Puchong BK3 objection/complaint on LRT, until LRT station cancel at there.

you circle 3 place so small, how to built depot? 

1) only 250-300 meter long ? can demolish Theme Park?  demolish some commercial high raise building there  for LRT viaduct route to there? some is hospital, school, (belong to Sunway group)

2) center 1, is Sunway NEO mix development in construction

3）wrigh small 1 only can fit bus i think.

this building is built after Monorail is complate, just like Kerinchi Station people already know & accept it