Seems like a good question for https://bitcoin.stackexchange.com/ – I think segwit was originally a single push, where the first byte was the version, and the rest was the program; with that push being between 2 and 41 bytes; now the version and program are separate pushes, but rather than having the program be 1-40, it’s 2-40…
EDIT: How could a 2 byte witness program make sense? - Bitcoin Stack Exchange suggests it’s to avoid ambiguity on how to push one-byte (OP_1 vs 0101 etc)