To the people wondering who cares about what MTV has to say about anything, or who would watch it and why Pratt would address it at all : The entire thing is a venue for promotion and to increase brand awareness of whatever they're showcasing. The awards themselves are meaningless essentially, but serve to get popular figures on a stage in order to sell advertising for MTV and increase brand awareness of whomever they have in the spotlight (in cases where they need to remain relevant, like singers and other performers).

MTV makes some money and the performer gets some more exposure and retains their place in limited spotlight. Chris Pratt and the professional industry folks around are totally aware that his attendance to this type of event is largely irrelevant. He's a part of a successful and loved television show, two huge movie properties (GOTG/Avengers and Jurassic Park) and has a loyal following based on his charming personality.

So, why would he address it? He doesn't have to and the fact that he made prior obligations to a great cause is only icing on the cake to a continuous campaign that nearly all public figures engage in. He uses his fame to promote a cause at the small expense of whatever exposure he would get from attending an award show. Doing that charity work is rewarding to him and increases the value of his brand to his fans (that's not to say he's doing this for purely selfish reasons, he seems like a nice guy that way).

Despite his perceived public persona (an everyman and sort of a dullard), the guy has been in the industry and is surrounded by people who are in a similar situation. He knows what he sacrifices by being absent at award shows and weighs that in the face of doing good charitable work that can also be used as a means of publicity for both himself and the organization he's working with.

TL;DR Pratt is smart, MTV is important but charitable equity is more important for many reasons.