Thank you for all answers. There was some magic there... Right away after I have sent this post I got the idea, and offered this role with limited timeframe i.e.:"Who would like to fill this role for next 2 months?" - and soon I had 2 volounteers.
It looks that the real "Why?" for them to not take it, was that everyone was affraid to "stuck" in the role, or they didn't wanted to be the bad one, who took the role and then gave it away (causing problem for the others...)
Anyway thank you!