User blog comment:Miskos3/Fairy Tail Episode 241: Tartaros arc: Devil Reincarnation, Review/@comment-4933386-20150711153839/@comment-7418318-20150711173455

The only holes I remember Virgo digging are so tight that some of the .... larger profile members like Droy and Elfman wouldn't fit through. Invading Cube through a hole that allowes only going in single-file seems a really bad idea. And getting the wounded and unconscious ones through such a tunnel would also be problematic.

On top of that Cube isn't mere stone. For all we know Lucy summoned Virgo, but she failed to dig into it and we just weren't shown on screen.

Besides, they were hard-pressed by the Tartaros Troopers, so maybe she just didn't have an opening in which to think about something other than not being killed.