Finally, found the reason why Microsoft is bringing all its apps to other platforms...
One of the reasons, many Android or IOS users dont switch to WP is the so called app gap, mainly the absence of Google apps.
Microsoft, by releasing its apps to IOS and Android is giving a chance to these users to try their apps and probably say:" This app is better than what I have". So these users may start using MS apps, and then they will have no reason not to switch to WP, where they can get all MS apps built in!
For example:
An Android or IOS user, if downloaded the Cortana app. He will use it and find out it is actually better than Google Now /Siri. He will say:" Wow, Cortana is better than Siri, maybe on WP it will be even better!" And so hopefully he switches to WP.
I think this is MS strategy.