This is about nothing but the Senate's Byzantine rules - the ones Mitch McConnell and most other Senate Republicans won't even consider changing - and the idiotic budgetary assumptions on which these rules are based.
We've talked about this before, and I wish we never had to talk about it again, but the long and short of it is this: In the Senate, you can't pass a tax or spending bill with a simple majority unless the Congressional Budget Office "scores" it as not increasing the deficit. The CBO is staffed by Keynesians who do not believe cutting marginal tax rates spurs growth, and as such they assume that every rate reduction worth a dollar increases the deficit by a dollar. This is idiotic beyond all measure, but these are the rules and the Republican majority shows no interest in changing these rules.