I've been wondering if they were going to drop last generation consoles just as they did with the original game, which was originally announced as coming to 360 and PS3 but which they dropped along the way.
Dying Light would have required 5 engines for that gen instead of just 3 for this generation.
For Dying Light 2, previous and current gen machines are similar enough that the engines are the same with just some minor tweaks to the rendering path, so 3 engines used regardless of whether previous gen consoles were supported or not.
Additionally, reducing asset quality enough that it would fit into 512 MB (X360) or split 256 MB CPU, 256 MB GPU (PS3) from what was used for 8 GB consoles would have required significantly more work than what would be needed for current gen (16 GB) versus previous gen (8 GB).
Basically supporting PS3 and X360 for Dying Light required significantly more time, money, and manpower than supporting PS4/XBO would for Dying Light 2. This isn't to say that a PS4/XBO release isn't work, but it's quite significantly less than attempting to support PS3/X360.
Considering Dying Light's release was delayed from a 2014 release to a 2015 release, I'm guessing by that point they were scrambling hard just to try getting the PS4, XBO, and PC version to a point where they could release it that they probably entirely stopped working on the PS3/X360 engine sometime early in 2014 in order to ship something without a potentially further half year or more delay in release. That's assuming that the PS4/XBO version was ever hitting internal milestones in a timely enough manner that it was feature complete and a port to PS3/X360 was even started.
Heck, if they are using the Xbox development environment, then they are potentially just working on 2 code bases as the Xbox dev. environment now allows you to compile to XBO, XBS, and PC with the other codebase being for PlayStation consoles.
Regards,
SB