Terri, the only reason I can think of is, even though a rooting plant needs moist soil, Geranium cuttings shouldn't be kept constantly wet. Some MG soils are heavy so additional mediums such as Sand (not beach) and/or Perlite should be added.
Also, I find rooting cuttings in water much easier than soil, but when a cutting is rooted in water, they develop water roots..then when transferred to soil, it's like starting over..IMO it's much easier rooting a geranium directly in soil. Water well, but let dry a bit between. Don't set in direct south or west sun..medium light is best until roots establish.
Did you use Rooting Hormone? I don't but many people swear by it. It's about 3-4.00 a bottle. Just dip end of cutting in hormone, then place in a soil..
Which brings me to pot size..when you repotted in soil, what size container did you use? For one or two cuttings, a 3 or 4" pot is fine. Nothing bigger..Larger pots equal more soil and more soil means more water and too much water equals root rot.
If possible, take another cutting and root in soil. You need not add plastic around a geranium when rooting. Treat as you would a succulent, well-draining soil, bright light, smaller pot. Good luck, Toni