Because when you are talking about $30 million a year versus $50 million a year, that is an extra $20 million a year that can't be spent. Even if you think the Sox are a $200 million payroll franchise in the future, that is 10% of your payroll EXTRA that is tied up in one player that can't be used. A $20 million payroll slot can get you a pretty good player. While they might be able to absorb it early, it would come back to haunt you in the later years of that deal right when your younger players are hitting their primes and getting expensive.