Multiple reasons. First the "Buy America Act" requires that a percentage of assembly be done in the USA to preserve jobs here. Massachusetts added to that making it a part of the contract that the assembly needed to be done in this state. This meant that the rail company needed to build a manufacturing facility for rail cars, which would be rather large, within the state.

The second reason is that the state, by law, is required to accept the lowest bid for any contractor that checks all of the boxes in the contract proposal. In this case, CRRC came in well-below any other rail manufacturer and also promised to build a facility here to get a foothold on rail manufacturing in the USA.

Some of these laws are there to protect the state's funding and assure it is spent well, and also to preserve jobs. On paper this looks great but has not always worked.

For example the current "new" fleet of MBTA locomotives was bid by a well-established European company as well as Motive Power in the mid-west. The MBTA preferred the European company but it was a little more than what MP bid. MP sued under the "Buy America Act" and as the low bidder so the MBTA was forced to buy from them. Those new locomotives are now being cycled out of service for work at a regular rate to fix something broken or worn. As a result of that the MBTA had to pull a handful of older locomotives that had been mothballed, get them rebuilt, and pressed into service.

So it is not always the MBTA's choice. The various labor laws and laws that control cost and bidding often tie their hands.