Blame Aethelred the Unready, who ran away to Normandy and let the Danish king Sveyn Forkbeard take the English throne for a year in 1013, and then his son Edmund Ironside who lost the Battle of Ashingdon in 1016 to Sveyn's son Canute, yes that Canute.
After Canute and a couple of his sons had their turn as King, the Anglo-Saxon lineage was sort-of restored with Edward the Confessor, but his mother was actually from Norman stock. By that stage the royal house of Wessex was thoroughly entangled with the Normans and the great Earls of Wessex, East Anglia, Mercia and Northumbria (all established by Canute and with powerbases of their own) weren't happy about it, but the manouvering to get their candidate – Harold – onto the throne after Edward's death opened the door to the other two claimants, Harald Hardrada and William.
And the Pope was just taking the chance to back a winner… 😉