Meanwhile a Windows application compiled 25 years ago still runs today. A Windows application compiled 30 years ago runs with a very small compatibility shim you can ship with it as a DLL.
I have no specific insight into decision-making in open source organizations, but given that the resources they're working with are several orders of magnitude less it's not surprising to me that they sometimes end up having to choose between stability and new functionality. (While I personally prefer projects towards one end of that spectrum, I'm happy that different projects have different values for other people.)
*Usually, not always.
A GTK 2 application compiled 25 years ago usually still runs today, too. Just because GTK 3 is available doesn't mean GTK 2 stopped existing.