@MarshaBradyo
I don't know Cornwall at all, so I can't really compare. (sorry this is a post of guess rather than data)
I suspect at least part of it is luck. Cornwall, I think always stayed low, so less likely to have cases "rattling around" and able to spring up.
Perhaps weather too. Blackpool can be windy and wet. No snow though, if you're interested. A great frustration as a child. 
There's a certain number of indoor attractions, and people don't tend to go down to the beach in the way they do in Cornwall.
Cornwall I'd think of sitting on the beach/surfing/water activities. That's my impression rather than knowledge though.
Blackpool tourists are more doing things, taking a tram trip, visiting tourist attractions etc.
Blackpool, in nice weather people do do outside things. But a lot of the tourist things are like the Sandcastle (indoor swimming centre), Sealife etc.
Even things like the Pleasure Beach which is outside, I remember the cleanliness and safety when I went there. I'll put money on them not wiping down, queues being not socially distanced, possibility of sitting next to a stranger. I'd like to think I'd be wrong there, but I've heard a few stories from friends that went.
I'd guess the difference between Blackpool and Cornwall is luck. But there may be more to it than that.
People in Blackpool are often self employed. They can't afford to think "I coughed a couple of times, as I do every year, I need to isolate for 14 days" because that is 14 days of no money. When the season's as short and unpredictable as it is, a fortnight is a long out time. So there may be an aspect of less likely to isolate. I don't think that would be significant though, and it would be more given to tourists who would take it back.
One other thing, and this is observation. I saw a certain amount on the news about Cornwall being unwelcoming to tourists. Now I don't know how true this was, but it may have put people off even if it wasn't true.
Blackpool I think was actively advertising people to come.
I'd wonder perhaps whether it was more the people who come in bringing it.
Blackpool tends to have a lot of visitors from Manchester for example. So when Manchester was coming up in numbers, that probably would bring it in.
The visitors are also often in the 20s and 30s age bracket. So more likely to be asymptomatic and also the age bracket that came up over August.
I don't know how that compares to Cornwall though.