Simply put, there's a distinction between a "pure game of chance" and a "game of skill". Lotteries, raffles, luck-dips etc generally require a licence from a licensing authority (gaming or lotteries commission), whereas in the UK (and in Ireland) a game of skill (where the entrant has to exercise some degree of skill based on personal knowledge or semi-complete facts) is permitted without a licence. That's why there's always a tie-breaker or (quite easy) question to most competitions. Because What Hi-Fi is beamed into lots of countries with lots of different rules, I presume they don't take the chance on being fined in foreign jurisdictions.
That said, there is a line of jurisprudence that says the question in a game of skill to make it a game of skill mustn't be too easy, so as to make it a sure-fire cinch for everyone to be a potential winner (e.g. what colour do you most associate with oranges - (a) orange; (b) purple; or (c) photocopier).