He would dominate the debate stage and suck up all of the oxygen, causing all of the other opponents to have to share not even half the time. Not seeing any reason not to do it.
The last time he skipped a debate and planned a counter event, it did not work out well for him. He went from leading to losing the Iowa Caucuses. Rubio had the momentum (essentially the “winner” as he did “better than expected”)...then of course Rubio self destructed at the NH debate blunting and negating all of that momentum, re-splitting the opposition to Trump among multiple opponents.
“The last time he skipped a debate and planned a counter event, it did not work out well for him”
I seem to remember he won that election, so it did in fact work out great for him in the end.