There were only two options that Junior had. (1) Use Richie to whack Tony even though nobody else in the family supported this move, and just hope it turns out alright in the end (hope the captains decide to keep Junior as boss - when those captains may in fact decide to whack Junior for whacking Tony as this would have been an unjustified murder).
(2) Rat on Richie to Tony (This allows Tony to begin trusting Junior again. And, Tony immediately increases the amount of money Junior makes every week so he sees immediate benefits.)

He decided...it was a safer play to go with #2.