Why is that?
I think there is a couple of reasons.
Maybe because habit is second nature. When PL/I appeared FORTRAN had already existed for almost 10 years. When some new technology/language appears you can start to disparage existing code base by calling it legacy code. But there is no reason to start conversion immediately. Especially it was big problem in 70s due to the lack of automated converters and other tools.
The next reason might be that there is no silver bullet. PL/I was attempt to create such silver bullet - general purpose language. They tried their best, you know the rest. =) PL/I was monstrous language. Frow Wikipedia article:
Programmers were sharply divided into
scientific programmers (who used
Fortran) and business programmers (who
used COBOL), with significant tension
and even dislike between the groups.
PL/I syntax borrowed from both COBOL
and Fortran syntax. So instead of
noticing features that would make
their job easier, Fortran programmers
of the time noticed COBOL syntax and
had the opinion that it was a business
language, while COBOL programmers
noticed FORTRAN syntax and looked on
it as a scientific language.
Also the "old folks" like FORTRAN evolve under danger of death and added features such as structured programming, object orientation, etc. That reduced PL/I's relative advantages.
P.S. Also take a look at that part of Wikipedia article already mentioned.