Satoshi probably left it out because making a Turing-complete transaction scripting language safe is more difficult. You have to prevent scripts from running endlessly while also allowing them to run long enough to be useful, and you can't let them access too much external data or they might become invalid after being valid for a while and really screw things up. A Turing-complete transaction scripting language would be really cool, though. If done well, it could be used to add all sorts of new features (even new crypto) in a backward-compatible way.

I don't know much at all about Ethereum, but it looks like their scripts can access info about blocks, which seems dangerous. If this was possible in Bitcoin, then a reorg could cause many fully-confirmed transactions to all of a sudden become invalid by accident, without even any attacker. This is very undesirable. Maybe Ethereum does something to fix this -- I don't know.