Because constitutional amendments are hard to pass, by design.

You need a supermajority in both the senate and house of reps, and then you also need effectively a majority vote in 75% of the states (so 38 states now).

There is a sufficiently large number of smaller states that it is no longer possible to change this.

There are some courses that cover the basics of US constitutional history, which might be instructive:

https://www.khanacademy.org/humanities/us-history/road-to-re...