My best guess is that Nvidia is unhappy with how OpenAI is fishing for compute with its competitors (Jensen had some opinions on the AMD-OpenAI deal when it was announced). If this actually becomes a feasible reality, it gives OpenAI (and co) negotiating power - which is bad for Nvidia

Nvidia might have wanted more exclusivity/attachment. And OpenAI still seems to have no problem raising money. So maybe there was just a commitment mismatch

Pure speculation though