The Reapportionment Act of 1929[0] is the culprit for this problem. Up until then, the size of the house grew with the population of the country.
It was one of a series of early 20th century foundational changes to American government that opened us up for the legalized corruption we're suffering from now.
The 17th amendment[1] is another, which allowed for the direct election (and thus hyper politicization) of Senators.
With 50 states, the country would still have 100 senators, but special interest groups would have to write checks to thousands of state assembly members to influence the election of Senators, rather than just writing a few dozen to influence the Senators directly.
[0]https://en.wikipedia.org/wiki/Reapportionment_Act_of_1929