Plug-in: Roland Sound Canvas VA.
Utilities > Cakewalk Plug-In Manager says it is in the 'VST Instruments' category.
Insert > Soft Synth menu has it in the 'Uncategorized' sub-menu.
 
Why? What do I need to do?

Only VST3 plug-ins natively support plug-in category tags. Even then, some manufacturers elect not to use the feature. Cakewalk has categorized some VST and DX plug-ins in the database provided with SONAR. Regardless of the format, plug-ins may be added to an existing category or a new category at will. Refer to "Sort by Category" on this page for additional links about how to use the feature.

Well that was easy, lol.  ...and that's enough late night for me!  I'm out....  (and thanks!)

You realize you can easily re categorize any synth. I often get 3rd party vst's that need me to manually categorize. 

Cactus Music
You realize you can easily re categorize any synth. I often get 3rd party vst's that need me to manually categorize. 

LOL - that reply is equivalent to someone asking why their car won't start, and you tell them they can easily fix it, then walk away without telling them how. 

I would have included instructions but I don't have Sonar handy. But it's there in the browser somewhere with I think a right click on the synth in question.