Eastern Tennessee was largely pro-Union, while Middle and Western Tennessee were pro-Confederate. However, the Union forces were generally unable to keep and hold the area due to the mountainous terrain and the long supply lines from Ohio. Union strategy focused on dividing the Confederacy by seizing the Mississippi Valley and taking the capital of Richmond. Eastern Tennessee was not a priority. In contrast, West Virginia provided a buffer for the key Union states of Pennsylvania and Ohio, and had key rail lines connecting the Northeast with the Midwest. Taking over West Virginia was more strategically important to the Union cause than assisting Union sympathizers in isolated Eastern Tennessee.
In the case of Northern California/Jefferson, there is no reason the powers that be in Sacramento would permit its secession. In a similar vein, it is unimaginable that Texas would allow liberal, majority Hispanic South Texas to form a new state.