I presume they were waiting to see what happened to Solaris. When Oracle bought Sun (presumably the only other company who might have bought them was IBM) there were really three enterprise unixoid platforms: Solaris, AIX and RHEL (there were some smaller ones and some which were clearly dying like HPUX). It seemed likely at the time, but not yet certain, that Solaris was going to die (I worked for Sun at the time this happened and that was my opinion anyway). If Solaris did die, then if one company owned both AIX and RHEL then that company would own the enterprise unixoid market. If Solaris didn't die on the other hand then RHEL would be a lot less valuable to IBM as there would be meaningful competition. So, obviously, they waited to see what would happen.
Well, Solaris is perhaps not technically quite dead yet but certainly is moribund, and IBM now owns both AIX and RHEL & hence the enterprise unixoid market. As an interesting side-note, unless Oracle can keep Solaris on life-support this means that IBM own all-but own Oracle's OS as well ('Oracle Linux' is RHEL with, optionally, some of their own additions to the kernel).