Aside from what everyone else is saying about the semantics, there were also a lot of changes in the header format to make it easier to parse in hardware by ASICs. Fixed-length basic header, a single integer flag that indicates to routers when they need to parse variable-length options, a requirement that in the unlikely event they are used that those options need to be 64-bit-aligned, etc.

As long as they were breaking compatibility, all kinds of details were changed to make things easier on implementers.