Because they want a piece of the iOS ecosystem. Development for iOS is only available on Mac, which makes a Mac the most versatile development machine. You can do on a Mac what you can do on a BSD machine, which is almost the same as what you can do on a linux machine. IBM is funding Linux development directly with obscene amounts of money, they are to thank for a large chunk of useful Linux development efforts in the last couple of years. Apple does not give money to linux, but they contribute quite a lot of money to select open source projects that benefit linux directly, like MP-TCP.
IBM put the original NSA keys in Windows, Microsoft didn't even know at the time that Windows NT had the original NSA keys in it from the IBM OS/2 source NT is based on. You can be sure that IBM knows what's going on. Maybe IBM doesn't want it's employees to use the one operating system that doesn't have MP-TCP or advanced security features or a decent virtualization solution.
To be honest, the latter is the only thing that counts for companies like IBM. Whatever the bottom feeder's machines are running as operating system, all the company applications run in VMWare or Xen anyway, and thus they run on the company's linux servers.
Mac and Linux also can do virtualized application cloud services, delivered by a modern linux server cluster, something Windows can't do either.
IBM also uses Lotus Notes, they are not on the MS-Office crap, they need more robust solutions and they make them themselves.
These are the things that matter in modern companies. Whatever way you look at things from an enterprise perspective, Windows is a consumer operating system with an emphasis on media consumption and entertainment, the days that Windows was even remotely an efficient productivity platform are long gone, and the same is true for Office. The last version of Office that was a true productivity platform was Office 2003, and that version, still the most popular version right now, doesn't run on Windows 10 any more.
IBM is the most knowledgeable IT company in the world. They don't fall for the cheap cheesy marketing by Microsoft. Microsoft markets a featureless mobile browser à la standard AOSP browser like it's the best and most innovative thing ever. The standard browser hasn't played any significant role in Android since Android Gingerbread. Why would that be different in Windows. Everything about Windows 10 is meaningless marketing, there is no substance, no innovation. Cortana doesn't work in anything but American English, and requires extreme spying to even work. At least Siri and OK Google are now available in other languages and have spyware opt-out functions after legislation and feedback from the customer base. In Windows, nobody even cares, the market penetration of Windows on modern mobile devices with the professional and consumer markets combined is less than the market penetration of GNU/Linux on the old PC platform in the consumer market. IBM is not going to invest in that. Even Microsoft doesn't invest in their own platform any more lolz.