My thinking is money.
Although the the NLRT extension to Blatchford was said to need 6 cars, if they keep 12 minute peak headways, they can easily open that extension without any more trains. Right now they have 13 minutes of layover time, Blatchford Gate is maybe 2-3 minutes. In fact, I suspect the Metro Line is scheduled so that they could extend to Blatchford Gate at any time without changing the signup.
Operating the CLSE to Heritage Valley is likely possible with 2 extra trains. It's tight, but, 32 new Rotem cars would allow 9, 3 car trains for the Capital Line plus spares, and the numbers work for 32 SD160's plus spares for the Capital Line, allowing 8, 4 car SD160's for a total of 17 Capital Line trains, vs todays 15 trains. I guess if needed they could run a mix of 3 and 4 car SD160's if needed, or keep a few U2's for peak service.
One thing we don't know is how much these cars cost. I'd be curious vs. the budget how much these 32 cars will cost. It could be they only bought 32 if they came in priced higher than expected, or it could be they ordered less to help free up some money for bus replacement purchases. They did have options for 24 cars, so what I could see happening is that since the 32 cars will be in the 2027-2030 Capital Budget, they will produce a report in the next few years justifying exercising some of the options in the 2031-2034 Capital Budget.