Massachusetts is a large state that has a very powerful Governor with no term limits. Being one of 100 Senators would be a downgrade, particularly since he would immediately start out as an underdog for reelection.
Democrats tending to hold a veto-proof majority in the legislature diminish the significance of that office for a Republican and at some point, Baker may either want to move on to a different kind of job or find that the electorate/political forces are tiring of his incumbency (after his third or fourth term if not earlier, I'd imagine). Given that presidential politics are a no-go zone for him with the current iteration of the Republican Party, the realistic alternatives are the Senate and the House,.
I suspect Baker is gearing up for another run at the governorship in 2022 but I could see him interrupting his third term with a tilt at the Senate in 2024.
