The two party system is an improvement over the multi party system. For any political issue there are always 6 or 7 different policies to deal with it. If you have 6 or 7 different parties, Murphy's Law guarantees that each party will take a different policy, none of them will have enough votes to push their policy thru, and so nothing gets done. For historical examples, look at Britain and France during the 19th century. Britain was steady two party. France was lots and lots of parties, every party for itself.
In two party system the party leadership decide which of the 6 or 7 options to take (or invents yet another one more palatable to the party members) and when the vote comes, the entire party puts all it's votes on one option and it will pass. Things happen. Progress occurs.
To make this work, the party leadership needs to get all the party members on board. Good policies help, good leadership speeches help, but when push comes to shove, the party leadership needs to be able to say," Vote with the party and these good things will come to you. Buck us and these bad things will happen to you."
Used to be, Congressional leadership could offer (or deny) desirable committee assignments, and juicy pork for your district. And money and presidential support in your next campaign. Or money and presidential support to your primary opponent.
Now, not so much. I forget the details, but the good government types have taken away the leadership's absolute control of committee assignments. The Republicans outlawed "earmarks" special bills giving money to special causes. Nobody is sure that they even want Donald Trump's support in their 2018 campaign. And so, poor old McConnell tried hard but he couldn't get all the RINO's and rightwing screwballs on board for repeal and replace. Perhaps he could have made it with a few more carrots and sticks in his hand to bring members into line.
Next time the good government types are out there pushing some reform that weakens the leadership's incentives, maybe we ought to vote 'em down.