There were multiple reports Betts was seeking $400mill, with some going as high as $420mill. Even if that is just a negotiation ploy, it makes a ton of sense to turn down $300mill. What appears to have happened there was Dombrowski either was not allowed to spend more, or he simply used his Plan B, which was to sign Bogaerts and Sale.
Fans like to believe the greedy owners are always at fault, but the players are greedy as well. MLB is a $10mill business that a couple thousand people cannot agree on how to divide up amongst themselves without quarreling and threatening to quit every few years. They're all greedy, including the star players...