Because it is an insulting question in this context.
First of all why as a journalist, would you assume that a Palestinian representative is affiliated with Hamas? If there is any confusion, the question, "Are you a Hamas member" would suffice.
He isn't there to condemn a definition of violence that does not include the people he represents, against whom the vast majority of violence in the region has been committed and gone unmentioned.
What is so special about the barbarity that went on a few days ago? Are you not aware of the barbarity going on today, right now, in Gaza? Or 3 days ago in the West Bank? Or every day?
This is literally the Associated Press journalists asking MLK to denounce violence of riots in Watts in 1965, ignoring the fact that society had condoned and blessed 346 years of state sanctioned violence in the form of slavery, lynchings, Old Jim Crow and de facto segregation. It's like, "oh, now we're denouncing violence? Today?"
The world (including the BBC) has blessed (through commission or omission) acts of barbarity in the region for over 50 years. Keep the same energy. Don't choose his interview as the time to grow some morals and ask, "won't you condemn the violence??" Especially when we know good and damned well that those morals will be dropped the following day.