Because the verb "tend" is in interrogative mode, and interrogative mode needs the auxiliar always even in those tenses that don't use auxiliar in affirmative (present simple and past simple).
The same reason why you ask "What did you buy?" and not "What you bought?"
If you're actually asking the question, it has to be the second way. If you want a label headline for an article or something like that, you could use the first way, but only without the question mark.