Because it provides significant advantages over the 6 string that the 5 string doesn't quite manage. For example, being as light as possible, range of choice of strings, range of choice of bass, less of a need to mute effectively for recording, and simplicity when it's preferred, and so on.
I honestly don't find that with the neck being too wide. With 6 string I prefer 16.5mm at the bridge so the width is minimised, and keeping my thumb in the centre of the neck means that there isn't much of a difference for me between 5 and 6. I had been toying with the idea of getting a Sire M5 for reasons of GAS, but I'm finding it very difficult to justify to myself when I have a lightweight 6 string. I'm not really all that fond of the B string anyway, and which is the only point of the 5 string, so it's not really a priority for me. I tell myself that I have a 6 string if ever I need a B string. The 6 string also allows for various future expansion possibilities which the 5 string does not.
I think switching between 4 and 6 string also stops me from falling into a musical rut in the way i play.