A good programming font helps avoid confusion. For example, it should be clear if a character is l, I, or 1 without having to see them next to each other (also o, O and 0 as well as others). A font should also make reading code as pleasant as possible (though of course this is highly subjective).

When you consider that programmers spend most of their time reading code, it's not surprising that many programmers care about the font being used.