it's a simple matter of cost. However, many of us are using lipos. The last several flying sessions, I haven't taken any nicad/nimh, stricly lipos. I don't fly parkflyer stuff though, so they're large (and expensive) packs,.. 3S4P , 4S4P, 6S4P, 8S4P, 9S4P,...7800 or 8200 mah are the sizes (replace 10,20,and 30cell nicad/nimh packs I was using in planes from 3 to 12 lbs ). I rarely take packs out of planes anymore either, i'll fly 10 or 15 minutes, and plug into a charger while flying something else, or BS'ing in the pits (half the fun of the hobby is the comraderie anyway). With most of my setups, I get 25 to 30 minutes from the 7800 and 8200mah packs, so with this procedure, I've always got a good 10 to 15 minute flight worth of "fuel" on-board,..i.e. don't wait for charger to completely finish most of the time,.. and could go right back in the air for another 10~15 minute flight immediately if I wanted to. the big issue is cost,..for example, a 3S4P pack is $240 (to fly my 10-cell planes,..i.e. E3D, sport-s-stick,etc.), and it takes 2 of them for the 88" span E3D (almost $500 worth of battery in there). If you do the math, the cost per flight minute per charge is about the same (would take 3 $80 packs to equal one $240 lipoly), but most people son't want the large single expense.

In your example of parkflyers with the little 1000 to 2000 mah packs, they actually are very common these days.