The iOS one was just an app update in February, not an all new app.
MLB usually publishes a new app. The reason is that the Premium version is a paid app. A new app requires another payment. The iOS update was likely a new app looking at it technically.
I remember a couple years ago the MLB At Bat app for WP from the previous year worked for a bit. It was soon shut down however.
Disclaimer: this is how it used to be; I'm not sure how MLB does it now. I subscribe to MLB.tv, so I download the free version. The Premium services are then activated when I log in.
Now that I have an iPad (Mini 2) I'm looking forward to comparing the service on all 3 platforms this year! That's of course assuming that a WP version will eventually be released.