Simple answer: I just like how it smokes with virginia flakes.
Longer answer: There are several reasons. I already have enough pipes with larger chamber, and even if I love to sit all day sipping on a flake, a smaller chamber is sometimes what I prefer. But more essentially, it has to do with my preference for the mechanics of smoking flakes (fold & stuff) with a smaller diameter. I find that there is a significant difference that happends in just one or two milimeters below Ø 20 mm. That is just my own personal experience, but I find the pipe easy to pack, I like the control it gives me over the ember, making it easy to light, have an even burn, sip gently etc. All together, it makes for a better smoke, IMO. If you have smoked fold and stuff in a pipe Ø 20 mm or more, you will probably notice how the tobacco burns well in the middle, but not always at the edges; you might end up having to use your pipe tool to kind of break off the flake from the edge to get a more even burn, but I think just a few milimeters smaller makes a difference. There is also taste, which is of course a very subjective thing, but I sometimes think flakes tend to smoke sweeter in a less wide bowl, but yeah, different factors at work here, there is also deapth (allowing the sugars to caramelize as the bowl progresses).