I think Puma probably just offered her more money.
Occam's Razor answer right here.
A few of things could have happened here. First, Saucony may not be in great fiscal shape right now since the COVID economy is hurting a lot of brands. They may need to trim costs and sports marketing will take one of the first hits.
Second, perhaps Molly and her agent got too greedy? With her Trials and London results, they might have come to Saucony with too high a number. During negotiations, both parties could not come to an agreeable figure and both walked away. If Saucony doesn't feel that Molly creates additional demand for product, then it is not a justifiable outlay.
Lastly, Puma just offered a better deal. For all we know, Puma put something on the table that Saucony couldn't match but, Molly's agent gave them a chance to do so. In addition, Puma is re-establishing its technical running business after a long hiatus. They are building that business around younger faces (Warholm, Duplantis, and others) and they think that Molly is a bankable future star in this country.