1.10 was the "base" modding version most people started with. Also; there are way more plugins available for these who don't know how to make code edits and thats why it is the most version atm. 1.13 got some fixes and other possibilities (respec,..) which don't need to be edited manual, also it is more stable on realms.
To expand a mod to 1.13 you have to synch the .txt files from 1.13 with the 1.11 one and then make all changes over it. Also, if the mod have code edits, you have to get the offsets for 1.13 (which are mostly abit different) and make them yourself. I think there where 1-2 more things to do i can`t remember.