The current code contains a rather complex script template matching engine, which is only used for 3 particular script types (P2PK, P2PKH, multisig). The first two of these are trivial to match for otherwise, and a specialized matcher for multisig is both more compact and more efficient than a generic one.

The goal is being more flexible, so that for example larger standard multisigs inside SegWit outputs are easier to implement.

As a side-effect, it also gets rid of the pseudo opcodes hack.