Creating a ecosystem actually is a really hard problem, Microsoft grabbed deep in their war chest to make it happen and failed.
If Huawei wants to succeed with the Harmony OS, they need to better than Android in some capacity. If Harmony OS wants to be better than Android in some capacity, then it cannot afford to be just an Android fork.
Ecosystem creation is not that big of a problem, you just need to throw money at it. Anything that can be solved with money is not a big problem.
You're asking developers to reallocate resources from a money making platform to a unproven platform. Making that business case is a really hard thing to do here in the west. Having to already support Android and Ios.
Maybe in China it will be different especially because huawei choose to support Android app and have their HMS services on the Android platform.
Get the people first to use harmony with Android apps is a smart move. If you ask me.
I am interested in how huawei will create a Linux based HarmonyOS desktop distribution. I wouldn't be surprised Microsoft knew about this move given they are moving over more and more api to Linux.