I used to work at Eastern Mountain Sports (a New-England, USA, based chain of outdoor goods). Granted, I haven't worked there in over 15 years, but I know what you are talking about. It was very popular early 2000's up to about 2010. Whether due to fashion or pricing (like Gabriel C. has suggested), I also don't see too many combo jackets any more.
I'm not sad about the demise of this trend, though. I'm glad it's going.
The Zip-in feature is nice when you are in a casual setting, but, from a performance stance, you are leaving a huge strip of your torso uninsulated (or under-insulated) if you use the zip-in feature. My old jacket caused 2-3 inches (6-8 cm) of a gap where the insulation of the fleece stopped, and the zippers of the two layers went together. 2-3 inches might not sound like a lot, but, given that this was a strip running the whole length of the center of my torso, I never used the two pieces as one combination jacket when I was doing anything 'for real' outside.
It was also a problem of people mixing the brands. Even though almost all zippers in the USA come from YKK, exact lengths and sizes could vary enough to make mixing brands not 100% easy.
I vastly prefer to have a separate fleece under my wind / water shell, so I don't mind that this trend seems to have died out. If you want one lined jacket for everyday use, go for it. But I really think that the performance of two layers is better than the compromise that was the zip in fleece.