Likely the same reason why banks and other financial institutions still use COBOL and Fortran code written in the 1970s or earlier on archaic mainframes: Top management decided at some point it was too expensive to rewrite everything from scratch in some modern language for modern hardware, so they just limp along with what they have.

A 16-bit app written for Windows 3.x would almost certainly have to be rewritten for modern, 64-bit Windows.