Vokes55
The reason they didn't was they expected they'd end up having to run a split LHR/LGW operation as they have four daily flights on a BAU basis and couldn't make the move all at once, or worse move back to LGW entirely when things pick up. But you knew that. 😏 The examples you gave are mostly new entrants at once daily or less. Loganair got special treatment for domestic connectivity. Westjet as a new entrant didn't get enough to move everything in one go.
The fact they consistently keep applying for enough LHR slots to close their LGW operation shows how commited they are to LGW.