I think you've hit the nail on the head - kits dropped out of favour a bit in the 90s, and boots also removed themselves from anything other than their core range of medicines/beauty stuff/photographic stuff. I remember when they used to sell computers! The two just combined really meaning that there wasn't a viable market for their own brand stuff.
But the other kits that my dad used to do when I were a boy, John Bull, Tom Caxton, Geordie are all still going strong - probably something to do with the price at the bar these days and that we, as a nation, seem to be getting "craft-ier".
Just look at all the artisan bakers, home-made greetings cards, cupcakes etc that seem to be springing from everywhere.