What was wrong with using classical character sets for application programs?

Different character sets have very different architectures. In many, even simply detecting which bytes form a character is a complex, contextually-dependent process. That means either having multiple versions of the program code for different markets, or making the program code is much, much more complicated. Both of these choices involve development, testing, maintenance, and support problems. These make the non-US versions of programs more expensive, and delay their introduction, causing significant loss of revenue