I'm not a Ethereum core dev, but this is my understanding:

The EVM is tailor-made to accurately account for the expense of executing programs in a distributed environment. It has a highly heterogeneous addressing scheme. Every opcode has execution cost metering that has been refined over time. Each word is 256 bits to make 256-bit cryptographic operations easier. There's significant tooling around the EVM for writing and analyzing smart contracts. Other EVM-based blockchains would need to migrate in parallel. The skillset of "blockchain application developer" has coalesced around Solidity and the EVM.

Bottom line is that although WASM offers compatibility with non-blockchain tooling, the blockchain-specific needs of Ethereum are so much better served by the EVM that migrating is difficult.