Lots of reasons actually but to sum up:
1. the design of the phone - looks pretty cheap for its price
2. W10M is still a joke and when buying such an expensive device, you should have a consistent and smooth experience. Right now, it's not the case. I know the experience varies between devices but WP is locked down, like IOS. However, unless you have a hardware defect on an iPhone, the issues are consistent across devices which makes things easier to fix. and at least the fixes are coming. On WP, you could take 50 versions of the same phone, remove the ones with hardware defects and you still have no idea what phone is actually running WP best because they all behave differently with completely different issues. Being a supporter or a fan does not mean I'm okay to have a sub-par experience just because.
3. after 3 years with WP, the app gap is not getting any better. W10M Is trying to reinvent the wheel once again instead of trying to be consistent. When buying such an expensive and powerful device, you should expect the best ecosystem and the best apps to take advantage of the device you bought. Mind you, I'm not only talking about the number of apps here but also their quality. After using Android after years of bashing, I realised how obsolete the apps on WP were. Besides, people wants to make sure they will get the newest apps as well. Only IOS and Android can guarantee that at the moment. Spending $600+ on promises and expectations... sorry, I'll pass. I've been patient enough, time to move on.
But I'll be back... ?soon.