The addition of an airport stop wouldn't have made a huge difference for commuters. I imagine that cost was the main issue for the very small benefit it brings. They couldn't just move the stop because the airport stop actually serves the population of East Boston, so they would have to still keep that stop and just loop the line by the airport, thus an additional stop.
Adding about 5 minutes to every commute to/from east of Airport T "wouldn't have made a huge difference?" You are literally talking about thousands of commuter hours wasted every single day. Another 5 minutes is a huge deal on a line where the entire ride from Wonderland to Government Center or State takes about 20 minutes. And building tracks for the Blue Line into the terminal area probably wouldn't be any cheaper than just building a people mover -- it would just change the agency paying the construction bill.
The reality now is that few passengers take the blue line to the airport. Even before Ride-share the blue line only catered to a small percentage of passengers because of how much of a PITA it is to transfer between lines. And if you're attempting to do it with luggage and at rush hour, forget it.
Nah, the real reason is that the demographics for air travel from BOS and for using the MBTA have limited overlap. You're not going to use the T to the airport if you live away from easy access to the Blue/Orange/Green/Red/Silver lines, and that excludes the vast majority of Boston's suburbs and more generally BOS's regional catchment area. You're not going to get on the Commuter Rail in Concord and make three transfers to get to the airport. Business travelers (when that returns) who can expense parking/cab/ride share/limo aren't going to use the T to the airport. So in the end you're really just talking about students, the highly price-sensitive, and airport workers using the T to get to or from BOS.
Where the MBTA really screwed up was with the Silver Line. That line should have been a full blown, tunneled rapid transit line with an airport stop, thru Chelsea, Everett and ending in either Malden or Wellington's Orange Line stops. Instead they built a slow glorified bus line.
Yes, the T rivals Massport for incompetence. But the Silver Lie was never going to be tunneled rail to the Airport because that would require a dedicated tunnel under the Harbor, and that would have carried a ten-figure price tag. It is baffling how slow that bus moves through the tunnels, but the worst part is the stupid route the bus takes between World Trade Center and the Ted Williams Tunnel. It goes up the ramp from the WTC stop and... immediately sits at a traffic light at D Street. And then the bus goes most of the way back to South Station on the surface (and in the surface traffic) because the Staties won't share their special ramp to the Williams Tunnel. The route of the Silver Lie to the airport does actually make Massport look like a successful, functional organization in comparison to the T, I'll give you that.
They could and I agree that that would have been a better solution from a passenger point of view but I suspect that such a plan was proposed and rejected by ride-share drivers/companies. The current setup is much better for them than the scheme that taxis use. Taxis have to wait in the holding area, sometimes for a long time until they get a customer. Ride share drivers can come and go as they please.
The rideshare companies were unhappy about the changes. And it is much, much worse for the passengers, but no one at Massport really seems to care about that. No surprise there. Cabs are expensive because of rent-seeking by medallion owners. Period. It's certainly not because they provide a high-quality service.