The Southern states on the Eastern Border have a lot of advantages that the Northern States do not. As mentioned, you have the depth to recover from initial losses, and Ukraine and Moldova completely stand between ruzzia and Romania. Slovakia has partial defense through Ukrainian borders too, with most of the rest of Slovakia's border being covered by Poland. In order for Romania to be invaded, ruzzia has to either launch an entire naval attack in the Black Sea, something we have seen they are not very capable of, or invade all the way through Ukraine or Poland, both of which are very unlikely to succeed. Add in some major terrain factors, including large and difficult to pass mountain ranges and wide rivers, and Romania is pretty damn safe from ruzzia all told. Any attack would take weeks to break through just Ukraine, let alone into Romania, which would give NATO way more than enough time to send reinforcements.

Poland, Estonia, Latvia, and Lithuania, on the other hand, share direct borders with either ruzzia or Belaruz. There are relatively few rivers or mountains to slow an attack down, and aside from Poland, they have precious little land to trade for time and reinforcements. They are relatively flat, and the theory before the war in Ukraine was that a massed tank assault would tear through quickly if they were not well defended. Now we know that isn't something ruzzia can really pull off, but still. There is also the fact that it would likely be easier to pull off naval attacks from the Baltic than the Black Sea. The ruzzians do not have nearly the same force projection or reinforcement capabilities in the Black sea, and are limited by Turkey's control over the straits.

These factors above also work the other way around. Romania and Slovakia would be terrible locations to launch counter-attacks from because you would have to chase the enemy through Ukraine. But Estonia and Latvia are within a one week assault range on St Petersburg if the initial attack goes well. In the Black Sea, Turkey actually houses many of the strategic ports and airfields we would need.

None of this is to downplay the importance of Romania as a partner in NATO, and I am happy you are part of it. But the positioning means the Northern Flank is more positionally advantageous for both sides, and thus receives more forces.