I think you're spot on, it is easier to prove (or prosecutors have more experience prosecuting) financial crimes. To give them credit-- they at least tried to get Holmes for defrauding patients too, but she was acquitted of those charges [0]. Her partner, Sunny Balwani, was convicted on all counts, including defrauding patients [1].
[0]: https://en.wikipedia.org/wiki/Elizabeth_Holmes#U.S._v._Holme....
[1]: https://en.wikipedia.org/wiki/Sunny_Balwani#United_States_v....