Because no one wants to rinne tensei a decomposed corpse. No one wants to see rins ribcag. literally.
It was too late for that... And before you say it, no, nagato didn't master rinnegan enough to do that when zetsu first introduced obito to him... nagato only developed that around the time of yahiko's death.
Bro I love Obito but he's full of shit really after being brain washed by Madara and then realizing that his best friend was a piece of shit for not keeping a promise letting him damn near die in vain. Obito was almost another Nagato just smarter and when mad genuine. But What if he brought Rin back he knew that she would have wanted to go back to Kakashit and he would have let her die all over again