That's not my decision, but I can give some "outside-looking-in" comments.
If you're talking about including them with the program, licensing intellectual property (IP) from companies costs money. This would increase Sonar's price - in some cases dramatically depending on how much a company values its IP. This would particularly affect upgrade pricing, as these plug-ins would likely appear first in upgrades.
The point of plug-ins is that users can choose what they want to use for particular applications; if Sonar upgrades cost another couple hundred dollars to include specific plug-ins, and people didn't want those plug-ins possibly because they'd already paid significant money for licensed plug-ins from companies like Waves and Universal Audio, I think those customers would not happy.
The Nomad plug-ins aim for a vintage character, and I assume they were included to add that element to Sonar.
If Cakewalk was to develop its own line of plug-ins that parallel what's already available and make them optional-at-extra cost, that would take away development time from the core program itself and creating new devices like Z3TA+ that don't have equivalents from other companies.
That said, it's clear that Cakewalk does choose to do plug-ins that cover the same territory as classic hardware devices - like the Console Emulator, Tape Emulator, and CA2A. I'm confident that the Bakers will make the correct decisions as to which plug-ins are worth pursuing, and which ones aren't.