I have had a P99 in 9mm since 2006, and I have no intention to sell it. Many others have come and gone. I bought a CZ-75 SP-01 to get into competitive shooting and ended up using my P99 for my Black Badge Course (having 1000's more rounds through the P99 was a definite contributing factor). The SP-01 has now been replaced by a Q5 Match SF. I have sold all but one of the Glocks I bought after the P99 (as a comparison), and that's only because it's a G21 in .45ACP and I have yet to replace it with a PPQ 45. I simply prefer the Walther over the other brands, for many of the reasons the other posters have mentioned, and my own.
The after market for the Glocks is vastly superior as are the custom options, which is why it has been so successful, along with their solid reliability. Glocks and M&P's are sold in large numbers due to their price points and reputation. Walther has a deeper history, but is a handgun you buy and shoot, not buy, shoot, modify, customize and shoot more. Just a few of the reasons it may not be as popular.