Because today it has become more obvious that keeping the protocol strictly focused on being money only, "specific speech platform", is imperative for its success. Apparently, this wasn't as obvious enough at the time of the genesis block to make satoshi refrain from taking that liberty.
However, the genesis block example is not comparable to the "jpegs on the blockchain" example, as that is orders of magnitude more hurtful than the famous gensis block inscription.
Today though, if you asked me whether I'd rather that satoshi had never done that inscription - so it wouldnt be used as precedent today - or i'd rather accept future transactions bloat the blockchain with arbitrary data, sure I'd wish he hadnt taken that liberty.
