Only Adobe insiders can answer this.  It appears they have chosen to only allow their own filters to access CUDA processing.  Many third-party plugs DO or CAN use the GPU (You can tell, because some of them have a "Use GPU" checkbox, such as many BCC effects, Looks (in preferences), etc.), they're just being hobbled by the application.  Many of the same filters ARE using the GPU in Ae.

I have put in my feature request.  Have you?

https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform

Hmm... maybe an Adobe employee would be willing to explain the reasoning behind this. 

I use quite a few third-party plugins, but I tend to stay away from them in Premiere because it slows playback down to a crawl.

You may have told us, Jim.  That doesn't mean you're necessarily right now does it!

From the Red Giant web site...

Paul – This is from Stu Maschwitz’s blog, and may answer your questions:

“Both Adobe Premiere Pro and Final Cut Pro bypass their own plug-in SDKs for their native 3-way color correctors. They use window configurations and graphics drawing routines that third-party developers don’t have access to. On some systems this can make UI interaction for third-party effects with Custom UIs slow. In the case of Premiere Pro, the slowness can be bad. Real bad.

Have you noticed that Premiere’s own 3-way color corrector has never been ported to After Effects? This is one consequence of the Premiere team’s choice not to use their own plug-in SDK. Another is that third parties cannot provide a fluid custom UI experience within Premiere Pro.

After Effects, on the other hand, “eats its own dog food,” and has no effects that don’t use the public SDK. This means that third parties can create excellent user experiences within After Effects. The benefits to us users are obvious — just look at all the amazing plug-ins available for After Effects.

Premiere Pro and After Effects actually share the same plug-in SDK. This is amazingly cool, because it means that, for example, you can start a project in Premiere, use Colorista II all you want, and then move the project to After Effects, keeping all your settings. But despite this shared architecture, plug-ins like Colorista II sing in After Effects and bog down in Premiere.

Red Giant has is committed to working with Adobe to resolve this situation. We love Premiere Pro and feel that it and Colorista were born for each other. The playback performance is amazing. We’ve done the best we can with what we have. If you try Colorista II in Premiere and find the performance lacking, please consider contacting Adobe and asking them to improve the performance of Custom UI plug-ins written to their own SDK.”

Different subject, I'll grant you, but my understanding is that Stu was taking about the general SDK implementation - and this may remain the underlying reason for Looks 2 not having a tighter CUDA integration.