obi-juan88 I'm not one of the Devs, so take my words with a grain of salt unless someone corrects me, but simply put an LTS kernel needs to maintain support for a "long-term" hence the name LTS, long-term support. Kernel 4.19 is only supported till about December 2020, but the LTS kernel that Solus currently uses, 4.9 is an LTS that is supported until at least January 2023. So just on that basis alone, it makes more sense to provide kernel 4.9 to users since that one will receive support for a much longer period.
If you are curious where I got this information it is provide over on the kernel webpage here:
https://www.kernel.org/category/releases.html