There are multiple problems inherent with switiching to digital. One, is for a screen of that size a 70mm film print is going to provide significantly more resolution and clarity than a digital presentation would. Another issue, as has been pointed out by other posters, is that a digital projection system would require a completely different set up with multiple projection sources arranged around the dome.
There's really not an ideal solution here. They could make a brand new set of 70mm prints, but that's expensive and would run into the same problem after several years of constant use. The other solution is going digital, but creating a new set up for digital projectors would also be expensive and would result in a loss of resolution from the current version.