That's because he's too lazy to do it. Everyone who has that much knowledge would probably know that a sentence should start with a capital letter and that "I'll" means "I will" and "youre" means "you're". If he doesn't know then he could have spelled "your" instead of "youre". Plus, it's just a tweet you don't need to write the perfect grammar like you should in writing your research papers.

Probably because he might be the one who will get attacked..