Nobody in his right mind would have refused to enter Super Six Tournament.

If they were invited (Bute)... or if they didn't had real problems, visa or whatever, I think the problems where real (Bika). Looking at Bika's wife you can question his sanity tho.
Here's only the financier reason among the other indisputable reasons(e.g Ken Hershman Quotes) some more fresh than other (see at bottom of the page)

I'll write down Bute's purses and you can't decide for yourself how much bravery and how much business were in the mind of the guys who entered the S6.
Note: Only about 60% of those money entered his pockets

Alejandro Berrio
19 Oct 2007 - 400.000 $
William Joppy:
29 Feb 2008 – 700.000 $
Librado Andrade:
25 Oct 2008 – 600.000 $
Fulgencio Zuniga:
14 Mar 2009 – 500.000 $
Librado Andrade:
28 Nov 2009- 1.000.000 $
Edison Miranda:
17 Apr 2010- 1.200.000 $

Bute will be making in 3 fights with Showtime more than in his entire reign as champion(period with biggest earnings from his pro boxing career) the sum of 6.000.000 $ (2.000.000 $ per fight)
In his first fights of his pro career he has only made like 5.000 or 6.000 $




Ken Hershman comments on not include Bute in the Super Six originally:

“I’ve addressed this issue in the past. In my view when we had to go about creating it and structuring it to get five promoters and six fighters on the same page. To me, the way to do it was with the existing relationships that we had. It wasn’t to say that I didn’t think InterBox or Jean Bedard or Lucian were trustworthy. I did. It’s just that I didn’t have the depth of the relationship I do with some of the others. We had to make some tough decisions and we made them. In terms of regrets, I think no and that everything happened for a reason. When we put the Super Six together I thought we had some really good fighters and some really intriguing matchups and it’s all been really fascinating. And now we have the opportunity with Lucian to take him and put him in with the winner of the Super Six for what would be the greatest fight in super middleweight history. We’re just really looking forward and not back.

PS: Does any1 knows how much his purse was against Brinkley ?

 

I read somewhere that his purse was close to 2.000.000 $ against Brinkley witch is hard to believe.