Because you're only beholden to the people who live in your ward, you only really have the interests of those specific 10-50 thousand people (less in smaller cities, more in larger ones) in mind.

This, it turns out, really harms the capacity for governments to make decisions that make everyone better off on average but might make any given small region of people unhappy (say, by building a powerplant or a garbage dump).

Of course, there are benefits to this kind of representation, and some kind of mix is always desirable - but the gist of it is people are more willing to make regionally-disinterested decisions if their interests are aligned across ideological rather than geographical lines.