The problem is that they're paying millions of dollars to advertise "Booking.Yeah" and when viewers type that into their browser, it will take them to a search engine with results for Booking.Yeah.
Then, Booking.com has to pay money to the search engines (like Google) to have their website appear at the top of those results, which costs them even more money, unnecessarily, every single day. Also, they're losing traffic to other results on the search page like the first organic (natural) search result that leads to bookingyeah.co.uk (not owned by Booking.com).
It was never a good idea for them to use Booking.Yeah in their commercials, but now with the new gTLDs, it's an even worse idea since consumers will think they can type it in more often whereas before they may not have tried to type it in.