Simple. Bret Hart had a contract that gave him the right to do what he was proposing. He was also willing (and this was largely backed by Wrestling with Shadows) to drop the belt to A) anyone but not HBK b) To anyone the following RAW. He'd been there for 13 years, he deserved the respect to have his contract honoured. The fact also is, the bloke turned down a MASSIVE contract from WCW in '96, to stay loyal to McMahon. He signed a 20 year contract, but when the WWF ran into trouble financially, they negotiated for him to leave.
Aside from the contract, most importantly, and as proven by Wrestling with Shadows, McMahon gave Hart his word. A man that goes back on his word like that would throw his own mum under the bus.
There were a lot of reasons why Hart wouldn't job to HBK in Montreal. It was NOT just a "this is Canada and I'm egomaniac about that". He'd been accused of sleeping with Sunny by HBK, according to Hart, HBK had said he would not do a clean job to Hart, well prior to this. HBK was very much an antagonist in Hart getting to the point where he insisted on his creative control clause being enforced. Again though, the worst thing, aside from the contract, McMahon broke his word to a guy who'd given him 13 years. Aside from that, it threatened the legitimacy of wrestling to a degree, wrestlers were conditioned to jobbing/working as per their scripts, they knew largely what would happen in terms of results before going to the ring, here that entire policy was completely ignored. Imagine if Hart hadn't insisted on losing, but instead went to the ring, performed shoot tactics on HBK, and utterly refused to lose... you'd have absolute anarchy, and you'd have a situation where the entire industry's ability to protect the 'work' part of wrestling was in contempt. The fact though McMahon lied to him till he was blue in the face, after 13 years, and all the concessions Hart made for him, that's unforgivable in my book, if I was Hart, I'd feel exactly the same way, and I'd have decked him too.
