This language is more simple than C, but more structured than Basic. The objective in creating this language was to compile compact code that would run efficiently and safely as a virtual machine, like a very lean Java.
I've taken this reference from online and could not get the original author on Board but I'd like to quote it here:
Actually, IBM attempted for PL/I to "wax" the 5-letter c-word programming language ( and fortran and Lisp as well), and its PL/I was BUNDLED (read "FREE") software with the S360 systems in the late 60s. Several major companies (Sears for one) bet their IS business on this "new programming language", and most got out of the PL/I business almost immediately. Why? Because the compiler that IBM delivered produced such inefficient, buggy code that no one could get their software to work.
(I interviewed at Sears in 1971, and they were looking for ASSEMBLER programmers because PL/I had failed them, and IBM was trying very hard to get out of the COBOL business, so their COBOL support was not too hot).
PL/I was definitely IBM's "language of the future". I believe that the IS community was not ready to deal with a language of PL/I's elegance and power, and the hardware was not really up to the job either. Memory was still way to expensive, as was disk. Also, the 360 was not a stack-architectured machine, and if any programming language needs a stack, PL/I is the one.