Why Rome?

How come Rome succeed in conquering so much but not cities like Sparta, Athens, or the other ones in Italy?

When I read about the Greeks or the Etruscans, it always seems they had their home city and some colonies but that none of them really wanted to dominate everything, or at least to expand too much. They won a war, loot & destroy, but didn't expand.

If Rome had been like them, it would just have conquer a couple of extra cities and then stop...

I read that it was because they were always attacked, so each time they protect themself by destroying/assimilate their opponents... But that's true for everyone : the sabins, the volcis, the latins... they were in the same situation, but only Rome manage to expand this much. It's like the others wanted to remains in their geographical area.

And on the Greek side, the only one who succeed, Alexander, didn't have a strong superstructure to protect his empire, so everything split as soon as he died.

So why Rome was different? Is it because of the Senat? Or their religion? Or the way their society was organized? Randomness?