The "old" camp that has been with the ecosystem since the late 90s, and the new camp that came in with the post-dot-com/browser-war cheap and easy access to LAMP servers.
The former knows Linux from the kernel up, where the desktop is for the most part a way to deal with things that can't be represented in text.
The latter knows Linux from the desktop down, and want those shiny graphics everywhere.
GTK2 is of the old group, where tweaking and maximizing personal efficiency was king.
GTK3 is old the new group, where presenting some kind of top down design experiences is paramount.
Hell, it may not even be limited to Linux. Look at how Slack is getting all manner of attention, even though at its base it is IRC recreated. But now you can plaster every line with emoji(?!).
Damn it, i should not be feeling like a grumpy old geezer. I'm not even half way to retirement...