Good question, friend. Just off the top of my head, I would say they were waiting for the proper season. Jack must have died in the fall...between August and November, and when Ennis went up to visit the Twists, Mrs. Twist was wearing a heavy sweater (in the movie though). I would guess they were waiting for the spring thaw, because you can't bury anything up in that neck of Wyoming until after it. The zigzag road to Lightning Flat was muddy, which we found out during the Roundup that it was muddy and impassable almost even in June.
'Nother thing is that Mrs. Twist moved carefully as if recovering from an injury, so maybe if she had been indisposed that might have kept them from doing the deed.
And perhaps they had been at loggerheads about it so there was an impasse.
And maybe Persephone and Demeter figure into the equation too.