My experience has been that there are different reasons to do them and that's driving uptake.
There is much more of a social trend for them in the US and Canada than the UK. We are much more concerned about privacy and how the information might be used in the future than American and Canadians.
If you go look on forums, one of the things people from the US and Canada are doing when they take tests is to find out if the family myth about being indigenous North American is true or not. It seems to be one of these urban myths that everyone has this story passed down in the family and it's quite commonly something that doesn't turn out to be true. And then there's a certain disappointment from not finding that. There's also a lot of mixed race people trying to find their roots. If anything being very white western European can actually be viewed as rather boring. Unless you actively looking to 99% Irish so you can go back to Ireland which is also a thing.
In the UK people who do tests still seem to fall into two camps rather than the 'just for fun' of the USA and Canada. They are either people who have done extensive family history for many years through the paperwork and want to verify their work OR they fall into the category of having a know family mystery that they are trying to solve - a rumour of someone, knowing someone is missing and trying to find them, or trying to establish parentage. The ethnicity stuff is actually a lot less in the mindset of Brits.
It's an interesting contrast between the two. It means if you are British and have ancestors who had siblings who followed migration pattern, then you are likely to get a lot more hits from across the pond than within the UK.
This isn't necessarily a bad thing. I've been able to solve mysteries about the origins of both DHs family and my Dad's family through the DNA which the loss of Irish records has otherwise prevented. It just filled in a gap.
In DHs case we didn't know where this man had come from - we suspected southern Europe but this turned out not to be the case. In my dad's case we knew it was an Irish connection we just didn't know whereabouts in Ireland. So it didn't prove an ethnicity we didn't know.
Overall it's proved that areas of research which were weak correct. Although there is one part of my mum's family I think is now questionable. This isn't really a huge shock. It's a woman who had kids from two previous marriages from much older men and my ancestors was supposedly the daughter of the third husband. Both the mother and father in question clearly had very colourful lives from the other information we had about them so it perhaps would have suited the man to take on a woman who was already pregnant for his own respectability and obviously it would have provided her with more financial security. Everything suggests it was otherwise a successful marriage.
I think the idea that it's just about proving how white you are doesn't reflect what I've seen. Go have a good look at the Reddit for ancestry. It's full of people being excited to find non white European ancestry. And slight disappointment at finding out the opposite! Of course there will be some that will try and do a test to prove the opposite - but that's a bit of a risk because unless you have done years of extensive work before you probably don't have a clue and might easily be surprised to find some Jewish in there for example. And there's then nothing you can do to then change that. Except lie.
The number of people who find they have expected matches is really high too. There are so many mysteries that are much more compelling than the ethnicity. Between DH and my parents, I've found four areas of the family within second cousin range of someone who 'shouldn't be there'. In DHs case it explains a lot of my MILs upbringing. My Dad has illegitimate cousins from one of his uncles (we don't know which one) and DH has second cousins who we have isolated to a second of the family. That last one was quite sad as it was a lady who had put her DNA up trying to find out who her father was. She died fairly recently but I've talked briefly to her daughter. I was able to tell the daughter she was descended from a one of several brothers but I could tell which from the result I could see. Seeing as it was her mother's dying wish to know, I think it has helped close a book. It's just a shame her mum didn't live quite long enough.