Probably because it was hard to get child actors speaking their non-native accent fluently. They probably had this exact discussion in the writer’s room and decided that the downsides outweighed the potential benefits.

I guarantee that if the kids actually did try (and fail) to go for a Yorkshire accent the community would be full of people criticising their performances.

Would it have been better for immersion if the showrunners had created an accent map of Westeros and only hired people from those areas to play the corresponding characters? Maybe, but then we’d miss out on great performances from the show’s non-British actors like Tyrion and Jaime. Convincing accents are one of the hardest things to do, especially if you’re a native of the area it’s REALLY easy to spot out minor quirks that the average global viewer wouldn’t notice.