The enemy of my enemy is my friend. Roth was wily and perceptive enough to know that Michael's ambitions included matching and surpassing/replacing Roth as the foremost casino mogul/legitimate "gangster" in both Nevada and Cuba. So it was in part aggressive self-defense. Keep Michael off balance and worried about what's going on in NY and maybe he won't see coming what your real plans are.
Also as Roth admitted, Michael's murder of Greene rubbed Roth the wrong way. So there was also a little personal revenge involved as well.
For Michael's part, I think it's fair to say that he also, similar to Roth, saw the Pentangeli-Rosato dust up as small potatoes and wanted to cement his power/foothold in Cuba/Nevada before dealing with Roth. This was before the assassination attempt on Michael but after that it was even more important to Michael that Roth underestimate him and think that Michael was blinded by the money that presumably the two could make together.
"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming
Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.
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