Where does the Rambam say that it only occurs in Israel? In Hilchos Tumas Tzara’as 11:6 he writes:
???? ????? ?? ????? ???? ?????? ???? ???? ???? ???? ???? ????
The Sefer Hachinuch in Mitzva 169 writes:
??????? ???? ???? ?????? ?????? ??? ??? ????? ????? ?????? ????? ???? ?? ???? ????? ????”? ?????? ????? ??? ?? ?? ??? ???? ????? ???? ????? ??????? ??? ????? ????? ???? ????? ???? ?????? ?????? ???? ????
See also Berachos 5b:
?”? ????? ????? ????? ???? ?????? ?? ???? ?????? ?? ?????? ?? ?? ??? ?? ??? ?????? ????? ????? ???? ???? ??? ???? ???? ???? ???? ??? ?????? ?? ???? ?? ??? ???”? ?? ?? ??? ??? ??? ???? ???? ?? ????? ?? ???????
??”? ??:
?? ?? ??? ???. ??”? ???? ???? ??????? ?? ?????? ???? ????? ???? ??? ???? ?????? ?? ???? ??? ???? ?????? ????? ??? ???? ???? ??? ?????? ?? ????
?????? ??:
?? ?? ??? ???. ??’ ??”? ???? ??? ????? ??????? ????? ??? ????? ????? ?? ??? ?????? ?? ???? ????? ?????? ????? ?? ??? ???? ??? ???? ?????? ???? ???? ??? ???? ????? ???????? ?? ??? ????? ???? ???????? ????? ????? ????? ??? ???? ?’ ????? ???? ??? ??? ???? ????? ??”? ?? ?? ??? ??? ????? ????? ??????? ??”? ??? ????
The Maharsha there asks:
???”? ?? ?? ?? ??? ???’. ????”? ?????’ ??? ?????? ????? ????? ???? ???? ???? ??? ??”? ??????? ??? ????? ??? ?????? ??? ??? ?????? ????”? ??? ?????? ??? ???? ??? ?? ????? ?? ??????
He therefore gives a different pshat in the Gemara. It should be noted though, that all these discussions are about whether the dinim are in effect, not whether it is possible to get tzara’as.
The question of why bizman hazeh we aren’t noheg the dinim has vexed several of the acharonim. R’ Yaakov Emden discusses it at length in She’ailas Yaavetz Siman 136, 137, and 138: http://hebrewbooks.org/pdfpager.aspx?req=1408&pgnum=131
The Tiferes Yisrael in his pesicha to Seder Kadshim discusses it and suggests that it’s because we only have ???? ???? nowadays:
http://hebrewbooks.org/pdfpager.aspx?req=37947&st=&pgnum=36 (footnote ?)
R’ Menachem Kasher discusses it and suggests that one can only identify tzara’as if he had direct shimush from someone who showed him what it looks like:
http://hebrewbooks.org/pdfpager.aspx?req=51491&st=&pgnum=208&hilite=