Aren't these chips all pin compatible?, so it doesn't make much difference - with the obvious exception being the USB capability of the 18F series (no point having a USB interface on board if the chip doesn't support it).
Because it's been available for so much longer (I remember when the ONLY EEPROM based PIC was the 16C84, and being really excited when the 16F877 came out!) - and the small instruction set makes it easy to use. The 18F series are more optimised for C, even the MicroChip application notes tend to be in C - which presumably is why they make a free compiler available?.