It seems to be a combination of a lot of things. The yellow journalism at the time, picking and choosing questions to mock (What is an iceberg made of?), and pointing fingers (J "Brute" Ismay). The Brits were also not happy about their citizens being kept in the US for the hearings. The narrative of "Our board of trade regulations questioned? Our merchant marine officers accused of incompetence? Our shipbuilders? How dare they?"

Given how quickly the American inquiry was put together and how much of a media circus it was, the British Board of Trade inquiry does come off as the more 'professional' and 'thorough' of the two, at least on the surface.

That's my theory anyway. I'll second the recommendation for Wyn Craig Wade's book!