In a recipe, yogurt toughens potatoes but whipping cream does not

I have a favorite tried & true recipe that I simply call ‘foil potatoes’. recipe link: http://www.evernote.com/shard/s1/sh/048c56cc-1481-48b2-89e6-88e897da651a/b0c036a6448b07e0a486f77acf2111cb This has been made multiple times with consistently good results. A few weeks ago I decided to try substituting the same quantity of home-made greek-style yogurt in place of the whipping cream. They did not tenderize at all and although being fully cooked, were ‘crunchy’ and not desireable.

As an experiment yesterday, I did another batch with part of it done with the original recipe using whipping cream,, & the rest with yogurt. All packets were done on the same grill side-by side with exactly the same time & temp. The packets with whipping cream were done perfectly & again the ones with yogurt were tough. My internet searches have not found any answers as to why this happened. In addition to wanting to understand the science of this difference, can anyone suggest changes I can make to have this substitution work?