This is very important for high-performance computing. These restrictions allows better compiler optimizations. Numerical codes are particularly difficult to port, scientific establishments usually do not have large otherwise idle programming staffs, etc. Fortran 90 changes the picture somewhat, the new language is very rich and complex, but you don't have to use or even know about all this complexity. The C standard requires only a basic double-precision mathematical library, and this is often what you get.
Case insensitivity eliminates bugs due to 'miscased' identifiers.
The lack of reserved words in the language gives the programmer complete freedom to choose identifiers.