The same argument that there has been for this since the 18th Century.
OK. This is a typical liberal progressive framing of the question. Everything is reduced down to ideological principles.
The reason that pure majoritarian rule is a problem is that you need to have a functioning country, or state, that isn't trying to tear itself apart. Imagine you have a 60% majority and a 40% minority who differ fundamentally in their world view and interests. One can solve that in a majoritarian way by just saying that that is too bad for the 40%. Different problems then arise from that, particularly if the two groups occupy different geographical areas. You set up a dynamic that is going to tend to drive the country or the state into splitting.
To deal with the tensions that this creates, you need to set up anti-majoritarian power centers within the state to force compromise and make the 40% feel they have some stake in the system. The alternative is the method used in the 1860s.
If there are no such forces at play, by all means, knock yourself out and let the majority get their way on everything. It seems to me as if what we see these days is nothing but these forces. Saying "yes, but we won the popular vote" doesn't solve this basic problem.