g77 supports use of /* to start a trailing comment. In the GNU Fortran language, ! is used for this purpose.
/* is not in the GNU Fortran language because the use of /* in a program might suggest to some readers that a block, not trailing, comment is started (and thus ended by */, not end of line), since that is the meaning of /* in C.
Also, such readers might think they can use // to start a trailing comment as an alternative to /*, but // already denotes concatenation, and such a “comment” might actually result in a program that compiles without error (though it would likely behave incorrectly).