Why the mixed response?
Because people have differing opinions on what constitutes a great challenge. What you presented was an exceptionally clear, well laid out challenge. However, the objective was just much simpler than is typical on this site. It's not a bad objective, it's just very simple. Many people equate simplicity in objective with poor quality, hence down votes.
Note that in my comment I said it's too simple to be an interesting challenge, I never said it was a bad challenge. My recommendation to post to the Sandbox was not saying, "This is crap, don't post here anymore," it was just a friendly recommendation to help you get a started with posting challenges.
Why the hatred?
No hatred whatsoever. I like to think that PPCG is a very welcoming community overall. While down votes certainly do not seem welcoming, it's a user's way of saying, "This can be improved"; there's nothing personal about a down vote. And though you currently have 4 down votes, you also have 7 up votes, which is good! That means more people like your challenge than didn't like it. Even some of the greatest challenge makers here get down votes.
Nobody here wants you to feel antagonized or unwelcome, and we certainly don't want you to give up on PPCG.
I recommend three things:
The Sandbox, where you can post your challenge ideas and get feedback. It can turn a good challenge into a great one.
Join us on in chat on The Nineteenth Byte. The folks in there are always happy to help with any questions you might have.
Don't let your first experience posting a challenge here shape your view of PPCG or your outlook on posting challenges.
Welcome to PPCG. We hope you'll stick around.