Northern Light
Superstar
It was not part of the USRP because it was not planned (or envisioned) at the time that was planned. It's like asking why the extension of the Yonge subway line was not built in 1954 when the first (Yonge) section of the subway opened!
To be fair, Steve Munro called out Mx at the beginning of the USRP for not providing for things like platform expansion; Mx already knew that would be needed based on planned service levels.
While it's often unreasonable to ask for 50-year foresight, in this case, the need for the further work was a known commodity and some mixture of apathy/lack of funding derailed (pun intentional) getting something bigger into the initial plans.