The bottom line with Betts was he wanted a LOT of money and he was not willing to simply sign anything. At the time, he had reportedly expressed a strong desire to test free agency. The Sox were defintely in a habit of spening a lot, and really, despite the lack of activity the past few offseasons, still are spending a lot.
It's real easy to say they should have just extended Betts, but it doesn't look he was was too willing to sign anything. The Sox had a very free-spending GM for the bulk of his career here, and Betts remained unextended. It's very possible the oandemic changed his mind in LA, especially since he signed the extension just before the start of the abbreviated season. Perhaps he was unwilling to be judged on a 60 game sample?