As the internal of the thylakoid membrane is hydrophobic the main factor is separation of protons and building of the proton gradient between thylakoid lumen and stroma. That is the main driving force for the ATP-Synthase. During the light-dependent reaction, protons are pumped across the thylakoid membrane into the lumen making it acidic down to pH 4. That nicely works out as the thylakoid lumen is small enough to be acidificated by a few protons inducing a strong force across the membrane for the proton gradient equilibration. Of course it is an interesting question if there is any further difference between thylakoid lumen and stroma, however I guess the proton gradient is the main reason for evolutionary formation of thylakoids.

You are perfectly right Dr Schmitt concerning chemiosmosis and thank you. But I should mention here that the internal (endomembranous space or otherwise thylakoid lumen) space of the thylakoid is not hydrophobic it is aqueous. May be you mean the inner part of the double phospholipid chain from which is made every biological membrane. But this part of the membrane is not participating in any contact with the aqueous environment around thylakoid membrane. A folding of a thylakoid membrane creates a thylakoid. My still pending question concerns the origin and composition of the thylakoid lumen's material.

yes- I just meant the membrane itself which prevents the charges protons from crossing the membrane and forces them via the ATP-Synthase. It prevents the protons to cross and therefore ensures the proton gradient.

There are also different enzymes found in the thylakoid lumen as compared to the stroma.

Here is a good overview:

Article Understanding the roles of the thylakoid lumen in photosynth...

Thank you very much for your suggestions.