It's the same reason BlackBerry refused to make BBM available on other platforms (although I believe they actually did just that a couple of years ago when no one even remembered what BlackBerry was, perhaps after the company name was sold to a different manufacturer).
BBM was released on Android and IOS in 2014 (six years ago), but it never caught on. It's still around in the enterprise space, but barely.
And the company name was never sold to a different manufacturer. BlackBerry allowed TCL and an India-based company (I can't remember the name) to sell BlackBerry-branded phones running BlackBerry software, but that's all.
