*based on what i've heard (ground reality could be much much different)*
Speaking in Indian army terms
There's a sanctioned amount of Nafri from Army, Airforce and Navy SF units.
Earlier there was some sort of rivalry b/w regular and SF btns of para regt so as to which bns will send its troops to fulfill their(army SF's) sanctioned strength. Maybe it acted a catalyst for the myopic decision of converting regular para to SF btns. Even the current GOC-in-C is a regular para offcr Maj Gen JS Nanda.
One more fact, Regular paras had changed their mandate to something that mimicd ranger regt somewhere in the last decade. Considering the fact that these guys have a lot of backing within the army circles (gen katoch and many others have mentioned this fact a lot) hence it was extremely easy to bring upon such changes rapidly while SF was totally neglected and heavily understaffed till date.
These regular para offcrs even made the sfts to run adventure programs for civilians lmao. Thank god this isn't the case anymore.
The same guys who totally dismantled Army's SOF capability trying to do the same with the other services... only to satisfy their egos is just -