Because, again, this isn't an issue of "the Senate and the President hav[ing] a different opinion", but the Republicans being unable to get their agenda implemented via the legislative voting process, and as a result threatening to close down government entirely unless they get their way.
They're third-graders throwing a tantrum and threatening to break stuff unless they get their way anyway after being outvoted.
And if what their voters want is destructive for not just the country, but the entire basis for governing it, that's even
less reason to support what the GOP is doing. This is why the "suicide bomber" analogy is so apt - they aren't just keeping promises to their electorate, they're threatening to tear down the entire thing unless their demands are met. Congressmen are not elected to focus solely on what their small congressional district wants at the expense of the
entire country, and wrecking the whole economy of the United States because of the truculence of a tiny subset of voters is dangerous and counterproductive.
The nation
cannot function that way, and if this is truly the way things are going to be, if the Republicans insist on holding everything hostage so they can get their agenda implemented no matter how many votes they lose, then we might as well pack it up as a country and go home.