Language evolves. Ð/ð made it into the very early Old English alphabet, but it didn’t last long and was supplanted by Þ/þ for both soft and hard sounds.
Bear in mind that very few people were literate at that time, and that there are very few, if any, words that were distinguished by the need of an ð (I mean, we get by just fine with “th” for both sounds), so those who could write, simplified.
S and Z have a similar kind of relationship, and we harden S in places we might otherwise expect a Z, just like what happened with Þ. One of, if not the main reason Z has managed to stick around in English is because of its use in loanwords from other languages, and we’re now so familiar with them we don’t even think of them as foreign. And likewise Z itself.
Language evolves. Ð/ð made it into the very early Old English alphabet, but it didn’t last long and was supplanted by Þ/þ for both soft and hard sounds.
Bear in mind that very few people were literate at that time, and that there are very few, if any, words that were distinguished by the need of an ð (I mean, we get by just fine with “th” for both sounds), so those who could write, simplified.
S and Z have a similar kind of relationship, and we harden S in places we might otherwise expect a Z, just like what happened with Þ. One of, if not the main reason Z has managed to stick around in English is because of its use in loanwords from other languages, and we’re now so familiar with them we don’t even think of them as foreign. And likewise Z itself.