The pair dropped out of the Wales squad to play Bosnia-Herzegovina tomorrow due to injury, with Collison struggling with a knee problem and Collins said to have a groin strain.
"Two players withdrew again from the squad - Collins and Collison," Thomas told Radio Wales Sport. "They both drop out of the squad. Every time it's the same two players.
"It's unfair, because Chris Coleman, it's his first campaign, and he wants everyone available yet these two players yet again don't make the squad.
"If you don't want to play for Wales then why don't you just come out and be brave enough and say it, because Chris Coleman has to prepare his team for this massive campaign ahead."
Coleman has since called up Cardiff City's Robert Earnshaw and Huddersfield Town's Joel Lynch as replacements.