I need to add to the previous answers, which affirm that vous is polite and respectful, and explain why Mélenchon remains polite and respectful.
This is certainly part of the answer. I think that the only times I would expect a political figure to use tu in public would be when addressing their spouse or (their own) children on the dais with them (and maybe not even then), or in a funeral elegy for some person they wish to affirm was their intimate friend.
However it is important to see that even if Mélenchon did not wish to be polite and respectful (and I will not say he wished to), he would still absolutely not use tu, however much it might be considered insulting in some circumstances.
Tu is used between friends. It is intimate. Calling a person tu in public would suggest to people that the speaker knows this person in his private life and considers him a friend. Even if the tone and meaning of the speech is insulting, this would actually be softened by a tu, suggesting a quarrel between friends. Even if using tu were not inappropriate in a political debate, a suggestion of friendship and intimacy is the very last thing Mélenchon wishes in this case.
Vous may signal politeness and respect, granted, and tu can possibly in some cases be insulting, but I see those mostly as a consequence of vous signalling social distance, and tu intimacy. Respect would be upwards social distance, and vous can perfectly well also signal downwards social distance, but in the present case... it's just as much distance as possible.