Remember many cities were formed before the United States of America was even a concept and thus were inherently tied to the mother country. They were simple cities in colonies owned by Britain with a population dominated by British citizens. Not until the imperial crisis in the late 1700's did these colonies begin to seriously consider breaking away from the mother country and by that time city names were already entrenched. Also remember that many cities and states are named after neighboring native tribes or native names for the area e.g Ottawa, Ohio, Illinois. There was no real overall systematic approach to naming areas, it was a very localized and autonomous process.

that's why there is American English

That's not precisely the reason that US English is different from British English, it's much more complex than that. In some cases it's actually American English that is more conservative and continues to use vocabulary that is from the 18th century or earlier (e.g.: fall instead of autumn) long after British English has evolved, but in most cases it's simply pragmatism. There are a lot of new words in American English that didn't exist in the UK simply because they were native concepts to the Americas or were borrowed from French, Dutch and aboriginal languages that were here before English settlers.

In terms of spelling, Webster didn't really originate simplified English spellings, he just chose to use a different set of norms that tended toward a more logical orthography. In part this was motivated by a desire to show that American English was different from British English, but it was also simply that he was a proponent of spelling reform. He certainly wasn't the originator of this idea and he would have found many British spelling experts to agree with him at the time.

why didn't the British have more British names for Canada

Come to Ontario sometime. I grew up in Nova Scotia and have always thought that the place names there were pretty British in general (Dartmouth, Falmouth, Windsor, Halifax, Berwick, Digby, Truro, Annapolis Royal, Yarmouth, Cumberland, Inverness, etc.) but that's nothing compared to Ontario!

It's so over the top here that when I first moved here I couldn't help but snicker sometimes, there's a Humber River, Don River, Trent River, Avon River and Thames River to name but a few of the classics. Then there's Northumberland, Durham, Lennox, Leeds, Lincoln, York and Norfolk Counties before the municipalities were reorganized.

When the Loyalists moved here from the United States after the War of Independence they just went on naming things like they had been all along back home.

For starters, bear in mind what became the United States was essentially a British colony for 169 years. That's a long, long time. In that span of time, the British named a lot of things. NEW York, NEW Jersey, or Dover in Delaware, Bath in Maine, or Essex in Massachusetts. Some stuff was named after Native American or French lexicons, but you're right to notice many of the place names were English. In fact, in the aftermath of the American Revolution many towns and cities DID change their names, some of them even leaving British street signs when they changed them (sorta misses the point, right?)

The list of places in Canada named after places in the United Kingdom, however, is actually quite long. London, Ontario / Chelsea, Quebec / Liverpool, Nova Scotia.

Once the names were the way they were, people got used to them and left them.