.

Sometimes they check the winners profile and realise they've never once created a giveaway OR Suspect alt accounts

"Oh you've entered 300,000 giveaways but have never started1?" Yeah I'm blacklisting that person

Some people don't want 1 person to win multiple giveaways so as soon as that person won 1 giveaway the creator blacklists that person so (s)he can't join any other from that same creator, to give everyone a chance or something like that

many many many reasons.
rulebreaking - regifting, not activating, winning same game 2x+
bad ratio (taking and not giving)
quality of games given (too many free, overpriced asset flip/trash game farming, etc)
not playing games / only idling
too many lifetime wins
too big library
saying thank you (spam)
not saying thank you when you win (rude)
they don't like your name or avatar, or opinion on forum
the list is endless

Because they can?

Playing 15.8% of one's wins (for about 34 minutes per win) might lead some people to blacklist the winner. So their next GA might go to someone who wants to play it.

I only blacklist people who have 3 or more nonactivated or 2x win or "forgot" click :"received" (key is activated on account and active account!) AFTER to remind to do it.

I think I blacklisted winners for having a horrible ratio while not playing any of their won games. ¯\_(ツ)_/¯ but afaik some people do it to limit their giveaways as 1 win / user

How can you tell you were blacklisted?

I have met some users who do this because they don't want any user to have a chance to win against them again
Anyway, I don't think it's worth worrying about.
I stopped looking at my blacklist charts a long time ago. I think it's better for my nerves.

I only really blacklist for god awful ratios like 300 won 1 given 7 years ago or if they forget to mark as received even though you can see their active and have activated the key - I'm not tryna blacklist the entire website but I feel like weeding out a couple of the people who only take and those who've just given a negative experience is fair enough

Thanks for the replies..although I've been blacklisted a few more times since I asked the question. Ah well.