I would think they mostly disappeared because the same sounds could be represented by combinations of letters in the Latin alphabet which was used and recognized universally throughout Western Europe.
They didn't have standardized fonts those days, but the method of reproducing literal material by manually copying manuscripts still encouraged towards a simpler set of characters, resulting in much the same kind of thing. I Guess the slightly simple Brother Jonas was told that "oh come on, you once again messed up your ð and now that parchment is ruined, why don't you just write th instead?"
They didn't have standardized fonts those days, but the method of reproducing literal material by manually copying manuscripts still encouraged towards a simpler set of characters, resulting in much the same kind of thing. I Guess the slightly simple Brother Jonas was told that "oh come on, you once again messed up your ð and now that parchment is ruined, why don't you just write th instead?"