Because that was literally the only way they could convince the smaller states to join, and not have the whole thing collapse at the very beginning.
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...
Constitutional amendments are hard to pass. That doesn't mean they ought to be.
I/the poster is claiming small states should be allowed to be "bullied" by larger states since smaller states have less people, and thus, are less important.