Why wasn't he able to conquer Britain?

RAF air raids trashed all his landing craft while the luftwaffe couldn't get air superiority.

supply chains are hard, let alone amphibious supply chains. The British controlled at least some parts of the channel and the coast was fortified. then the RAF air superiority

Another reason was that they were not able to successfully stop war materiel being imported from the Colonies.

This precious stone set in the silver sea,
Which serves it in the office of a wall,
Or as a moat defensive to a house,
Against the envy of less happier lands.

You sound as if you WANTED Britain to be conquered, as if... somehow (snickers) ... we couldn't hold off Hitler by ourselves!

Newsflash: we could, and we didn't need anybody else's help except to invade THEM. Got it?