As someone that got introduced to C via SmallC and Turbo C 2.0 for MS-DOS, and avid reader of "The C User's Journal", one of the reasons was that ANSI C89 was supposed to standardize existing variants outside UNIX.

No compiler vendor back then wanted to give up on their own specific deviations, given their existing customer base, proprietary OSes and extensions.

So many of those vendor specific features got tucked away into implementation specific and UB buckets.