Is it? The dark side of the Force is not all about enslaving people and conquering the galaxy. Its as much about mastering your emotions as the light side, except the dark side uses emotions whereas the light side rejects them. A balance can be struck. Remember the Je'daii Order would not be dark or light, but a balance between the two, taking aspects of the light and aspects of the dark. Peace and emotion can exist at the same time. Just take a look at the Father.
Sorry, but this is not balance, this is totally light side. A darksider would simply say "Why the hell should we be peacemakers for the whole galaxy if we could use the Force to conquer it."
The Jedi actually started to support the Republic because they saw it as one of the greatest opportunity for lasting peace. Also, if I remember correctly, many Jedi were peacemakers outside the Republic.
And yes the Jedi Order claim to strive for peace, but half heartily. How can they full embrace peace if they advocated acts of war? And participate in them? The Je'daii Order would not do that. They would only ever raise a lightsaber to a Sith threat, and even then as a last resort.
But the Je'daii Order won't sit by idly. And dark side threats will be quelled before they can begin, and few will as the the Je'daii embrace light and dark. And if a Sith threat does manage to come into frutition, at least their fight will be first with the Je'daii Order, and not everyone else. This is the difference here, the Jedi Order always waited for the threat to come to them, they never tried to seek it out and suppress it. Because they feared the darkness that lurked away from their Jedi Temple. A flaw of the light the Je'daii would not have.
Well, that won't stop the Sith from enslaving non-Force-sensitive people and drafting them into military service to gain an advantage against the Jedi Order by numbers alone. Or, in general, building political entities much stronger than the Jedi Order. The Jedi choose, instead of building their own Empire, to support the nation which was closest to their ideals.
We also have to remember the Force has a will of its own. And as the Father says:
"Too much light or dark would be the undoing of life as you understand it."
The galaxy needs Ashla and Bogan - it cannot survive without both. This is partly the reason why the Sith threat always returns once the Jedi have destroyed it, because their destruction creates too much light in the galaxy, which needs to be balanced with dark. I'd even go as far to say that the Second Jedi Purge was the Force's way of redressing the millennia long shift towards the light side. Or at least the Chosen One was the Force's weapon to restore balance once it shifted to the dark. A Je'daii order would therefore be in harmony with the Force's will, a balance between light and dark. Therefore the Force would not see a need to redress an imbalance, and the Sith threat would never present itself.