I may be confused about this, but I think there was also a compatibility break from WP6.5 to WP7, and from WP8 to WP10.
It seems that eventually, mobile app developers had enough of this farce and abandoned Windows Phone.
It's funny that you can still run 1980s visicalc.exe on Windows 10, and CMD.exe supports 1980s DOS command syntax, but for mobile apps they decided that apps should be rewritten from scratch every year.
Yes, there was a break from early Windows Mobile (pre WinPhone7) because that UI system was really more or less a classic Windows GUI squeezed into a small screen and it sucked (unless you had a pen, but it was still a tad cumbersome).
I bought a cheap WP7 Nokia to develop on and the thing i remember most was that the UI was as smooth as Apple and not a stuttery one like Android of the same age.
Don't remember exactly why they left WP7 owners in the dust, but the biggest issue with it was that they fell for the Osbourne Effect and left quite a few mobile merchants pissed off about the amounts of WP7 devices left in stock that nobody wanted (and since those mobile merchants were often not the same as those with MS brand trust from the computer industry they felt no reason to give them a second chance).
While customers might've been a tad cautious, the biggest reason IMHO why WP8 died because few carriers,etc wanted to stock or sell them to begin with.
The number one rule of software and hardware should be never give your customers a reason to re-evaluate their original decision - and a breaking upgrade does just that. I had a friend who loved his windows phone 7 - but ended upgrading to an iPhone because of that.
The answer to that is Windows proper either ran on top of DOS or ran a DOS-compatible layer (Wow32). Windows Mobile 1.0-6.x and Windows Phone 7.x were based on the Windows CE (an embedded OS from the 90s) where as Windows Phone 8.x and Windows 10 Mobile were NT based with modifications made for ARM and battery life. Different kernels are the reason for the breakage.
Ah interesting. So compatibility issues from 7 to 8, and 8 to 8.1. But not 8.1 to 10?
I would really like to learn why these breaks were considered necessary at the time. In a practical sense it's irrelevant since WP is dead. But on the other hand, with the huge mobile market at stake, why did MS make these decisions that doomed it, and that seem obviously wrong to outsiders?
It seems that eventually, mobile app developers had enough of this farce and abandoned Windows Phone.
It's funny that you can still run 1980s visicalc.exe on Windows 10, and CMD.exe supports 1980s DOS command syntax, but for mobile apps they decided that apps should be rewritten from scratch every year.