Why would you expect a phone to run the full desktop version of an os? I though it was fairly obvious it won't since they use different architecture for the processors and it's a phone, not a computer. However, this is essentially what MS wants to do in the future.
That's naturally what people expect, when you give something the same name. They expect things with the same name to be the same thing. Windows RT was already too similar, so that name also raised false expectations. Many assumed RT to be some new edition of Windows, analog to Home or Pro, both of which have always run desktop software... RT couldn't.
The different CPU architectures aren't an obvious indicator either, because universal apps (or any other pure .net application) DO run on both ARM and x86. If a lot of software runs on any CPU (even MS Office ran on ARM), it will confound many why a lot of other desktop software would not.
My point is that while it's technically ridiculous to expect desktop software to run on W10M, people need quite a bit of information to understand why. Without that information, that false expectation doesn't seem so unreasonable. Just calling it W10 then enforces such false notions.
I'd also argue that MS does NOT (ever) intend to completely converge both OSes and end up with only one! As long as Windows must compete with low-end Android phones and tablets, MS needs a stripped down version of Windows that doesn't have the same hardware requirements as full blown Windows. Compared to W10, W10M runs on devices with half the RAM, 1/10th the storage, and far less power. It's not that you can't run full Windows on a smartphone sized device. It's that you can't do it at a price that competes with Android.
As long as MS must compete with lowest-end Android, W10 and W10M will remain separate and different products that share some identical components.