The Type 9s are still the same 74-foot-long footprint as the Boeing LRVs, and it's a one-of-a-kind design. It's an improvement over the Type 8s, but it's not something you want to base your fleet around for 30+ years. It's space-inefficient because the cabs and couplers are a fairly large percentage of the vehicle length, it has an inefficient door configuration, and it's only partially low-floor. The Type 9s were bought as an expansion fleet for the GLX, long before the Type 10s were in development.
Going to the Type 10s lets you use much longer vehicles that are more space-efficient: a single 110-foot car can carry as many passengers as two 74-foot cars. When you have high demand and constrained platform lengths across the system (and limited yard space), that's a big deal. Along with planned infrastructure changes, you can buy a proven, nearly-off-the-shelf vehicle that's 100% low-floor. (That means you can have true level boarding at all doors at all stops - and thus vastly smaller dwell times.) A smaller number of longer cars also means easier maintenance - you have fewer HVAC units, fewer signal units, etc to maintain.