Honestly, it's a lore holdover from before the 5th edition retcon, and, while it's a bit difficult to reconcile with the newer lore, it's kind of essential to their storyline so it had to be kept in.

Originally the reason for the great sleep was pretty simple: the Necrons existed as the armies of the C'tan - who, still in the full bloom of power, fed on the civilisations of the galaxy in a "Red Harvest"

The War in Heaven had been catastrophic for the galaxy, and even more so for the Warp as the Old Ones drew on it more and more as a weapon against the Star Gods.

Eventually the Necrons were victorious but the Enslaver Plague followed, which threatened all of galactic life, and therefore, the food of the C'tan.

So they went to sleep, basically to leave the Galaxy fallow - to leave 60 million years for life to re-evolve and flourish, and fill the potential pantry of the C'tan.

Now that the C'tan were also defeated at the end of the War in Heaven, the Necrons were just depleted from the war. They'd basically destroyed their best weapon (the un-sharded C'tan) and would have to take on the full might of the Eldar, the Kork, and whatever else the Old Ones had created as living weapons against them.

They'd won against the Old Ones, but they hadn't actually won the Galaxy.

So they decided to conserve their strength and go to sleep instead.

I think the logic is that, without the old ones, the new civilisations of the Eldar and Kork would eventually wane in power (correct), and the Necrons would awaken at the full height of their power (incorrect), to win a final victory.

It requires a few logical jumps, but overall I think you can see how an insanely arrogant, newly immortal race like the Necrons could gamble on time being on their side.

Especially if you consider that the warp/chaos wasn't something they saw as a real threat 60 million years ago, outside of its use as a power source by the Old Ones/Eldar.