I think the board at BlackBerry made the decision to make the BBM available to Android and iOS based on three compelling reasons, which are discussed below
BBM IS NO LONGER UNIQUE OR IMPORTANT
Which passing year RIM/Blackberry has seen its blackberry Messenger platform lose its holy grail status to its bigger cross platform rivals such as Whatsapp, LINE, Vibe, KakaoTalk. The CEO of Whatsapp had announced that whatsapp had over 200 million monthly users, LINE has over 150 million users in just two years of existence while BBM has about 60 (most of which are in Asia, Middle east and Africa, where the bulk of the blackberry users own fairly used blackberry devices) million users whose number have steady been on the decline.
Its seems the Blackberry has finally caught up with the fact that the consumers no longer see the BBM as a compelling reason to spend over $300 (based on the average selling price of a blackberry device) in acquiring a blackberry device, particularly when other devices offer a mix of better hardware, price and a good ecosystem.
NEW REVENUE SOURCE
In spite of the decline of the BBM mainly due to restriction to only Blackberry hardware, Blackberry could still stop the decline by making it available to other platforms. This would give it an option of acquiring more users from the sea of android and iOS devices being shipped every quarter. And by monetizing it by a monthly or yearly subscription charge (like Whatsapp) or by in app purchases (like LINE). The BBM can potentially top 200 to 300 million users within 2 years as it has already made a name for itself across different age groups.
REBRANDING AS A SERVICES COMPANY
The allure of becoming a services company just like Google, Apple (through iTunes), Facebook has become too strong for companies such as Nokia, and Blackberry to ignore. Google has made billions by making its services available to competing platforms, and so has Facebook, Twitter.
Blackberry has recently made its BES (Blackberry Enterprise Server) available on Android and iOS, bringing the BBM to the mix could also help keep Blackberry afloat in the unlikely circumstances that sales of the Blackberry 10 devices tank. So its a no-brainer that someone upstairs may have seen the wisdom in nudging the company in the direction its heading.
I believe the decision to take BBM to other platforms would pay off for Blackberry in the run long, if executed properly, and would serve as a revenue source for Blackberry.