Well, you can't say they didn't try. After one of the more prominent online OS/2 communities (OS2 World) delivered a politely worded petition with 11,000 signatures to IBM to make OS/2 into an open source product, the word has come back from IBM: Sorry, but no. Not happening.
To be honest, the more I thought about this issue -- especially after realizing how much third-party material also was tied up in OS/2's codebase -- the more I settled on the conclusion that it simply wasn't going to happen. The wording of the rejection letter itself hints obliquely at that: "...for a variety of business, technical, and legal reasons we have decided to not pursue any OS/2 open source projects."
I suspect the legal issues have become all the more tangled since their licensing of OS/2 as eComStation, which is sold as a closed-source commercial product, and is currently in a round of betas to release version 2.0 of the product (which ,according to their site, is "available early 2008"). In that light, OS/2 and its associated technologies are far from being "abandonware", as some people have put it, so it's not surprising that IBM would not exactly dive into a project like this with sleeves rolled up.
Even before that licensing deal there have been issues with how much of Microsoft's own code has been tied up with OS/2 (a story unto itself) -- and, under all this, the larger question of just how worthwhile and difficult the whole project is compared with providing transition paths from OS/2 to something else. IBM's stance has long been to encourage anyone running OS/2 to migrate away to something else, and while it may not be the friendliest stance to take, over time it's become the most realistic.
I feel bad for the petitioners, even though at this point I've come to completely understand where IBM is coming from. They don't see the payoff as being worth the effort, and they've already got a partner company wringing extra life out of OS/2 with their own compatible project. It was brave for the OS/2 fans to try, but maybe it's just time to move on. In the Bhagavad Gita there's the line, "You are entitled to your actions only and not their fruits" -- meaning that while OS2 World's people were perfectly welcome to send the petition, that didn't mean IBM was obliged to say yes.