Same reason .45 GAP sucks; Because it is the bastardized abominiation of a good cartridge that a few weenies couldn't handle.
The .40 is an exception to a common pattern, though-typically, short cartridges that come
after the original have less of a following and are dismissed (often with merit) by shooters familiar with the old version that performs better.
One of the reasons that the .40 became so popular is the sheer number of different guns that could be eaily adapted to accept it. The .45 ACP and 10mm require a large frame, and in many cases a double column version does not fit smallish hands. The .40 can be chambered in nearly any 9mm platform, and since wonder-nines were all the rage preceeding the introduction of the .40 Short&Weak, manufacturers capitalized on this advantage and flooded the market with 9mm-esque guns that had more power. Then enter the 1994 ban when people could no longer have 15 round 9mms, but most .45's only hold 7 or 8. At the time, the 10mm had become known as a gun destroyer because manufacturers kept trying to rechamber .45 pistols in this much more violent cartridge, thus there were only a couple choices in 10mm, and they had limited appeal.
The .40's popularity is attributable to many factors, but it's merits in and of itself are less than spectacular when compared with many other handgun cartridges. If one thinks about it, it truly is a compromise in every aspect; it takes the desirable qualities of other gun/cartridge combinations and sacrifices a bit of each to gain some of another.
The .40 also exhibits more uniformity from load-to-load, maker-to-maker; they all tend to be in the mid-400 ft/lb arena. 9mm and .45 come in low power (target) standard, +P and +P+. For 10mm, there is lite (.40-ish), average (.45 +p-ish), heavy (.357 mag-ish) and nuclear (just shy of .41 mag).
I've
had 6 .40's. I
have 1, and that is only because it is part of my S&W xx06 collection.