Actually, there are a number of typographical changes in various European languages that are now called letters but which, at the time, were properly ligatures.
The ampersand (&) is a good example. Technically, it is a ligature formed by the letters et: “and,” in French. One could argue that W is also a ligature, I suppose: double-U, which is not unfamiliar to Latin. Even the eszett (ß) in German is a ligature of long-S and z.
If I had to make an educated guess, I’d pin this one on scribes and printers. (Usually I’d say the Babylonians but this time they had a good alibi.) I doubt that literacy was widely spread in the early days, and most scribes were located in only a few centers, bound by a common religion, and they would be likely to copy a common style that they all used. The later printers probably created a simplified typography to match what the scribes were doing by hand, locally — after all, certain ligatures would arise more frequently in some languages than in others.
But why didn’t the scribes see fit to add diacriticals to their work in England? English has Germanic roots, and the Germans didn’t use many diacritics either; England was also invaded multiple times by the Romans, the Jutes, by the Norse, and the Normans (themselves an offshoot Viking colony). Beowulf doesn’t appear to have many diacritics and that was Old English. Perhaps English had too many outside influences for any one set of diacriticals to stick?
Wikipedia’s page on Old English suggests
There also don’t appear to be diacritics on the first page of Canterbury Tales by Chaucer, c. 1400 or so. The Great Domesday Book, a record made by William the Conqueror of all the towns and livestock, clearly seems to have diacritics in it (but I’ll be damned if I can tell you what language it’s written in). Ol’ Billy was Norman French, so that might help us rule out the Normans as an influence on typography.
So what could be the cause? Heck if I know. Given that Alfred the Great was an educated King, who made translations of his own from Latin, who compiled a Doom Book of his own (codifying laws, mostly), and who made a pilgrimage to Rome in his youth… I dunno. It could’ve been Latin influence. Or maybe the Babylonians.