Against a regular enemy, either they have an army somewhere you want and you bring your army to them, or vis-versa. In both cases, they have a limited number of troops: whoever's at the place, either originally or brought in ships. Once that force is deployed on the planet, destroying the ships they came in will help you reclaim the planet (orbit is the highest ground there is) and cut supply lines, but isn't the end of the fight. And that held true for 10,000 years
Well yeah.. if you want someone else's stuff then you kinda need to put boots on the ground if said stuff are on the ground.
If on the other hand you just want to keep your own thing, then a single Tau can beat a squad of custodes if both of them are seated in their own spaceship. Enough space superiority and said troops will die in transit.

Then the Nids arrived. And the Necrons woke up. Races to whom little things such as "limited numbers of troops" and "supply lines" are completely foreign. While a Hive Ship remains in orbit, it can birth an near endless supply of bioforms directly down onto the heads of whoever it likes. You killed one army? Cool, have three more. They simply don't stop coming until the Hive has eaten most of itself, or decides the planet isn't worth it. And if you drag the fight on long enough, they'll just set up breeding-pools and grow more directly on the planet's surface. Beat them in space, or it doesn't matter.
Its actually especially relevant for Nids. Battling them in space negates a massive amount of their advantage. Suddenly its no longer natural, bio-enginered killed versul upright ape. Its something designed to murder other things in space against something partly designed to travel long distances in space. In a enviroment where its especially hard, if not impossible to recycle biomass.

If anything, the Necrons are even worse than that. A single Monolith or Nightscythe can teleport in an entire legion of warriors directly from a tombworld, and keep them refreshed with reinforcements. A single scarab instructed to replicate can become a swarm of millions. The Necrons have no less than four different types of documented FTL travel/teleportation, almost all of which can function over interstellar distances and are nearly impossible to interfere with. Shoot them with orbital lasers, and they'll just come back again. Hell, a C'Tan shard might just fly into space and punch your ship to death. Beat them on the ground, or it doesn't matter.
If anything i would say Necrons are the only real exception to this. Mainly because its likely impossible to keep them out of your home unless your either Eldar or living in the eye of chaos.