I think a lot of Londoners with young kids give Cornwall a go because they hear from others about hotels like Bedruthen Steps where there is loads of clubs and activities laid on for kids all day long. Also, there are lots of mini farms and sort of low key theme park style places where you can go and just go for half a day with little kids. The weather is very changeable in the SW, so if it looks like rain, you can go to a farm and feed the baby animals and go on a tractor or whatever - then by lunchtime the weather might look better for the beach.
Also, I think people in London think, " Oh let's go to Cornwall - it's in the south, can't be that far.,." Only they realise, once they get to about Devon, that it's actually bloody miles and the motorway soon runs out and then it's twisty lanes all the way and traffic jam hell in peak season!
To be honest, the Devon coast (especially the Jurassic Coast) is just s beautiful. Though Cornwall does have some lovely beaches.
Personally, I think if it's wet or crap weather in Cornwall it can be quite depressing because lots of the houses are made of grey stone and the grey can be too much in places (eg Newquay). Devon and Dorset have prettier villages and cottages, I think.
The SE gets less rain than the SW so I don't think anyone is going to Cornwall for the weather particularly (unless travelling from the far north).
Also, we once got a train from London to Exeter and it took over three hours on what was meant to be a fast train! It seems that the SW has not had the investment in its railways / high speed connections that other parts of the U.K. have.