Yeah, I figured there are hairy problems about scripts depending on arbitrary other transactions. Maybe Satoshi gave up on making script really work because he didn't know if he could do it right. Best to get a solid foundation first.
Heck, having
transactions depend on other transactions in a block is a potential scalability problem because it makes it non-trivial to process blocks and update the UTXO set in parallel, especially in a lossely coupled manner. (ie multiple co-operating nodes)