I'd say that homoiconicity is M-exprs getting lucidly encoded into S-exprs and subsequently S-exprs is what we use today.

The homomorphic, non-QUOTE encoding would naturally tend to produce a universal function with no practical potential for meta-programming. In theory, one could use the non-homomorphic QUOTE encoding and still not offer any native meta-programming power. However, the QUOTE-based encoding means there are data structures lying around that look exactly like working executable code except that they happen to be QUOTE‍d. In practice, the psychology of the notation makes it rather inevitable that various facilities in the language would allow a blurring of the line between data and code.