They were not in the original GSC so they couldn't be in these besides they may not have had enough room for them.
HG/SS is mainly based of of G/S/C. And they didn't have the Sevii islands there so it wouldn't be in HG/SS.
the main reason the Sevi Isles were put in Fr/LG is because they wanted to have a place where Johto pokemon could be caught due to R/S/E's lack of many Johto pokemon and no easy way to complete the Nat Dex
also there would be no real point in having the Isles in HG/SS other than just another place to run around aimlessly
Youre right, but the Iles in HGSS could have been put in to have a place where Hoenn pokemon can be found, seeing as Nintendo is trying to rely less on those old cartidges, and the inclusion of the weather trio in HGSS. So you could of gotton more hoenn-nites. xD
But then again, i dont know how much more memory the games have/had left, so It may not of worked out.