The answer lies in logistics.
Have a look at the north eastern map of null sec:
|North East Null Sec|
The upshot is that althought those regions feel adjacent to the regular pilot travelling by stargates, they are inaccessible to Supercarriers even with Jump Calibration V. And since sub capitals are virtually useless in modern sov warfare in terms of destroying POS and structures compared to supercarriers (rough calculation: you need 320 mid range BS to match the firepower of 20 supercarriers) then launching a full sov offensive from Oasa/Cobalt Edge to or from Tenal/Venal is impossible.
So what is inbetween the core regions of the NC and the core regions of the Drone Russians?
|A big jump, but not as big.|
It is possible from Kalevala Expanse and Etherium Reach regions to reach Geminate and vice versa, as well Geminate is well within range of low sec for staging purposes. Both side want Geminate as a buffer to limit the options of the other for striking deeper into their territory.
As for opening a second front, both side would be forced to stage in low sec which may be OK for a short campaign, but really stretches pilots and resupply lines thin in the long haul as everyone likes the chance to be close to home for reshipping, R&R, industrial chores, home defence, etc.
Should CCP ever decide to finally open the Jove regions (i.e. Jovians finally died off / disappeared / enslaved) the dynamic would change the Geminate would no longer be the gateway region, but until then it shall always be contested.