There are a number of points:

If you build a networking devices, you have to have IPV4. You may or better should have IPV6, too, but without IPV4, you’ll have a bad start.

If you build an embedded system, like an “internet of things” sensor or actor, memory is at premium, so having to take care of a small and a large IP stack might be challenging.

Some systems are old that their software libraries don’t even offer an IPV6 option.

My current project uses IPV4 only, because I don’t need anything fancy. I just span a 10/8 network with 10.0.0.1 being the server. No routing, not even DNS. Using IPV6 for that application would be wasted.