Licensing fee's. Money period. Game titles and "content" are licensed for 5, 10, etc years a time and since PG has been licensed so many times it commands a hefty fee. In addition, the diff between the PG coding and those following were radically different.
SSI coded AG and then ported PG contents to that engine. And continued with the series so that why following games (except AG) are so popular. The AG/PG combo is also high value. There are also most likely contesting ownership rights, etc. which make it a risk.
SNEG companies, etc. Only have limited capitol and need to balance that vs the risk of game-success-failure vs the possibility that after investing in re-working, debugging, on newer OS's they will slapped with a cease-and-desist order by the courts whether valid or not. Funds my man. Usually can be fought and a compromise made but again funds vs eventual profit (or even just making spent monies back).
Annnnddd... Just like Hollywood. Rights are sold for a time block so occasionally those go stale and the games become available again. Likely what is happening now with PeoG and SG.