Because the US system requires 60 votes in the senate to pass most bills, not 50. This is the root cause of a huge amount of the dysfunction in the country.
The root cause is first-past-the-post winner-takes-all representation, inevitably leading to a dysfunctional two-party system.
Requiring a two-thirds majority for crucial bills is quite doable with proportional representation. It just means having to make slight compromises to end up with something most people will be happy about.
