That's a fair question. Michael Gundlach, the original developer of AdBlock, had this answer:
"Because I want to do this for everybody whether or not they can afford it. Because people who don't pay can still help out by spreading the word to their friends. Because sometimes in lieu of payment I get thank you notes, which are priceless. Because sometimes those who need ad blocking the most are those least able to pay. I could go on :)"
Michael has moved on to other projects, but his philosophy–that we need to keep our users first and foremost–continues to be at the heart of everything we do!
Read more here: Do I have to pay for AdBlock?