Well the explanation to that is a little tricky. Its not that pistols are tricky to maintain. Ive had and maintained the same pistol for over 6 years now and all I do change a few springs every few years. Its more of the usefulness factor mixed with the style of play factor.
More often than not, if you are playing out doors you will be using an AEG. Easier to aim and definitely faster and more accurate to get round down range with sufficient power to punch through bush. A pistol has its uses if you are speedy, switching to secondary is definitely quicker than reload of a primary. However that pistol is only useful if you are with in a sufficient engagement range, and that range generally is a fair distance shorter than that of an AEG. In a CQB environment a pistol is extremely useful as it can be used as both a primary and secondary.
"Its only a little bit on fire"