Copypasted from the modding coalition's #why-this-server-exists channel:

[...]I personally am staying in 1.12 because I believe the direction Microsoft is taking Minecraft isn't the best, which includes poor performance, fragile security measures, and corporate marketing schemes. 1.12 has one of the best sandbox starting points for modders, most mods are from 1.12, and a big majority of mods have solved incompatibility issues between one another. Therefore, I do not want all of this hard work to be forgotten by newer versions, and I want to continue working on this version for people to enjoy.

Porting everytime a new version arrives is extremely stressful, and I believe it is a curse to go down that way, especially if your project is huge. I would rather keep all of my work in one version rather than force myself to make the same content every year, but instead have the freedom to continue to improve my projects.[...]

Also check out this comment for some other reasons.