They thought they couldn't build hardware good enough to compete with other Android makers, but somehow convinced themselves that the solution to that was to put their less competitive hardware onto a less competitive platform.
But that is so wrong. Hardware was their strength. They could've sticked to what they knew best and over time do what's necessary.
They were slow. Really slow. Typing was so slow. Opening the camera app even more so.
Starting with Window Phone 8 everything was native code, even .NET, while on Android the first Dalvik versions didn't even had a JIT.
Only in retrospect. At the time, the internet was full of people who thought Symbian was Just Fine. And they had sales figures of millions of (mostly 12-key) "smartphones[0]" to prove it. And then this shifted to "Lost Causeism" when whatever MooMooMo platform never got off the ground.
[0] Okay, I had one of these and you could check your email or do a google search, but obviously the iphone it was not.