Because it has turned senate elections into a popularity contest they were never designed to be. We have the House for that.
The House has proportional representation by population, which gives individual constituents equal representation in our national government.
The Senate being popularly elected means if you live in a low population state you as an individual get extra representation for um reasons...
The Senate was supposed to represent the States themselves, the idea being that senators would legislate from a perspective of what makes sense in terms of federal rules to enable good State governance. Secondly to be a body a bit more isolated from the whims of the moment. Rather than react OMG OMG affordability! because there is a short term oil squeeze they be mindful about what might blow a hole in state budgets a decade later, ie while locking certain institutional investors out of single family real estate might sound popular right now, what will do property tax revenues over the next decade? (just a illustration not taking a position on the recent housing act here)
Now people will make various arguments about if it is easier or harder to 'buy' a legislator depending on their being selected by the state house vs winning a state wide election. My opinion is that in the era of big money and mass media it is easier buy a statewide election than to determine the outcome of 51 smaller races built around often very local issues, so you determine the outcome of the national election but opinions very. If you do believe otherwise fine, but then we should have simply eliminated the Senate rather than make it a popular vote; which I would also consider to be better than the current model and support!