The fundamental problem with Ark ships. They're just much harder to build versions of fully self-sufficient orbital colonies. Not only do you need to install the enormous thrusters, but you have to be able to operate without any inputs at all. No solar power. No imports of mass from asteroids or elsewhere. No increasing genetic diversity via swapping people between other colonies. What advantage is there in moving your orbital colony to a different solar system?
Slightly less chance of dying in very tiny solar system destroying effect. Traded to much higher risk of dying for some other risk or system failure. Anything affecting solar system likely affects also any ark on the way... And well inside solar system you might be able to get some support. Where as outside none...
Just what sort of population would enter that risk profile? Especially when there is little to gain for themselves.
Even if the sun explodes you're probably better off staying in the solar system and picking through the remnants than trying to make the trip out to another solar system.
