Because LLVM makes no guarantees about IR compatibility between releases. With the API at least, your compiler can catch the larger breaks when you upgrade, and they're likely to come with more doc/comments and change notes than the IR tweaks.
To be more precise, there is no guarantee at all about textual IR compatibility. Compatibility of bitcode, which is a binary representation of IR, is meant to be preserved across a very generous range of LLVM releases (but not forever).
