Why StumbleUpon not Support Dot TK domain name in Now?

I just sign up StumbleUpon account on last 2 days, But when tested to submit dot tk domains name, they show Warning!: Sorry, this isn't eligible to be added to StumbleUpon. I tested to submit with other domain name dot tk found problem still too on StumbleUpon site. I don't understand why they disallow all dot tk domains name in now!