Why is the EVM limited to saving 256 bits in storage slots?

I've seen this post:  

[Rationale behind 256-bit words in EVM](https://ethereum.stackexchange.com/questions/7382/rationale-behind-256-bit-words-in-evm)

But the answer doesn't quite explain why storage slot values are limited to 256 bits.

Obviously it puts lots of constraints on higher-level languages, such as Solidity, forcing them to come up with sophisticated and inaccessible storage layouts for dynamic values.

Technically LevelDB/RocksDB/etc can hold more than 256 bits (e.g. block headers), so why was the EVM limited to 256-bit slots?