Apple had to cut many of Siri’s original capabilities, as their size meant they became locked up in licensing legalese. Would Verizon have had better success signing deals?
I also think Apple had a certain aesthetic goal for Siri, that didn’t line up with the founders’ vision, which meant that struggling through licensing agreements was even less appealing. I think they wanted to bring focus to Siri, at least initially. There is a quote in the article that sums this up well: “Apple also seems keen to ensure Siri will be decent for many users, rather than genius for a few”.