Good question ! I think it would involve

**A wish to be able to fly**

Any long term rule not to fligh, or not to travel in space, will be challenged. 

People who have discovered engineering and technology also do science. A scientist will want to fly, to see if it can be done, to see what's up there. Many people *want* to fly.. on Earth, we apes had this wish, for centuries. Then.. you want to fly really high. You get into orbit. Or to the moon. To see what's up there.

**Atmosphere**

One practical hurdle may have to do with atmospheric layers. When your planet has a very thick atmosphere, it may be dangerous to travel through the turbulence.

**Gravity**

One other practical hurdle I can think of was found as answer #2 that is large gravitational pull, a big planet, or a relatively weak biology. It would be difficult for gellyfish to withstand the G-forces involved in leaving a planet.

**Size/weight**

I can add: when your planet inhabitants are technologically advanced and 2 inch tall, this will be a big hurdle to actually leave the planet. They may be able to fly, but building space-safe air tight constructs, as well as "powering up" for escape velocity may be out of reach.

**.. or religious constraints**

Your planet has a conservative clergy. One of the rules of faith is: you can't fly, only God can fly and the heaven is God's domain and you can't enter it, because if you do, you will fall and die.