Because the 1911 has a larger capacity, is faster to reload, is easier to shoot quickly (due to the short trigger stroke), and is thinner and easier to conceal.

Of course, 1911s have some disadvantages as well, in particular being less reliable than a good revolver.

I carry my model 66 occasionally and feel well-armed when I do so. But I compete with both my 66 and 1911 and am still quite a bit faster all around with my 1911s. YMMV.