Online FPS gaming has become casual and easily accessible, whereas before it was only, dare I say it, hardcore PC nerds that would be online.
The reason why games like CoD are so popular is because they are easy to pick up, accessible and enjoyable to a wider audience. You dont have to have mad skills to enjoy playing it and you dont have to have mad skills to feel like you are doing OK.
At the end of the day, game developers are businesses. They will track the gaming demographic and create titles that cater for that demographic in order to create profit.
Personally I dont mind any kind of FPS shooter because I love FPS games in general. Sure, I feel some require more skill than others, but I think you can have fun on most FPS games these days if you just accept them for what they are - a game, and a bit of fun.
I cut my teeth on Quake 3 deathmatch on Gifford UK servers. I had been playing FPS since Doom 2 but could never afford an internet connection. Wow - what a game Quake 3 is, and it has movement physics that were way ahead of its time.
Quake 3 takes some proper skill to master, from weapon choice to movement, to map control! The point with games like Q3 is that they rewarded skill and generally it was the most skillful players that came out on top in any given game. I think TTK is a big part of that. Example, in Titanfall you can down a pilot in 3-4 hits from the carbine which has a ROF of around 12 rounds per second - so even if your movement sucks and your overall play style sucks you can down anyone if you get the drop on them. Hence the hiding/camping/sneaky play style is rewarded with kills.
In games like Q3 and UT your opponents had a lot of health (and usually armour too) so you had to have the aim, movment, weapon knowledge and map knowledge to go toe to toe with them in any circumstances. Hiding and popping out when their back was turned would definitely give you the drop, but TTK and various weapons meant it did not ensure a kill. People playing Q3 or UT like they play CoD get raped, time and time again.
Also, in most of my games on Q3 - power ups were frowned upon (bar the mega and armour pick ups). You had no perks. No advantages other than ones you created yourself by map control / item control. Where I played for example (Gifford.co.uk - and Priest if you read this we still love ya man

), Quad lamers would get gang raped by the rest of the server because the community decided quad was not honourable.
Honour is something that seems to be lacking in modern gamers. It is all about the kill, and nobody gives thought on how that kill is achieved. The mindset appears to have shifted from playing to win with honour and respecting your opponent to playing to win, whatever it takes and then trolling your opponent if they lose or rage quitting if you lose. I feel this is a shame but indicative of casual gamers and, dare I say it, the huge increase in children playing online FPS games (even though a lot of them shouldn't be judging by the PEGI age classifications).
I think the console market has destroyed serious skill based FPS titles because people want to kick back for a quick blast - not invest hours and hours in developing their skill and getting better. Gaming is now a really broad church, and many types of people play online and many have busy lives. Jumping in for a quick game of CoD or BF4 before tea, or after the kids have gone to bed, or while the wife is in the bath is how gaming is done these days. Back in the Q3 days the players were those who played and did little else. Whole communities grew up around servers, and I am still friends with people I met playing Q3 FFA 14 years ago!
I can't say the same for CoD.
Another element I feel is a big part of the poularity is 'realism'. Kids love to use the guns the guys in the army use because it is 'cool'. Lets face is, a Q3 machine gun or rocket launcher does not inspire kids of today to go "cooool". With modern military actions gaining a high profile in the media it is little wonder the games today are designed around them. We know we cant own a P90, or M4 Carbine and most of us will never use one. But on a computer game we can pretend to be an elite soldier for a few hours and use the latest military hardware. If most of us are honest, we love it!!
My overall feeling is that online FPS has become casual and games now reward for killing opponents where killing opponents has become far easier.
Skills needed are lower, and the audience interest wider.
But actually I like how accepted gaming has become. When I started gaming it was stigma and something I would grow out of

It is nice that such attitudes have changed and I dont have to feel marginalised for being a gaming geek
As a lover of skill based FPS, I lament the last 6-7 years of clone FPS titles.
But as a gamer I love how my favourite hobby has grown and become more acceptable and I love how many more people can now enjoy it.
For me, the feelings I might have towards modern FPS titles are outweighed by the fact more people are gaming and having fun.
Afterall, isn't that what gaming is about?