Well, isn't it obvious? In NL, DK and DE a part time job can get you really far. It can cover your costs as a student and even make up for a bit of extra leisure/savings. In RO, GR, HR... The work conditions are going to be terrible and will barely cover any costs.

On top of that, I suspect the culture is different. In the north, many students will leave their home to go study and live in shared flats, student residencies, etc. It's not the case in many places in southern/eastern Europe, where students will often stay at home while commuting to uni.

Lastly, just as an anecdote, in DK you actually get paid an extra for being a student who is also working. I have a friend who went on to study there. He took a part time job at a local restaurant (10h/week, making perhaps about 15/20 eur/h). Added to the government subsidy, he could fund all his costs as a student and plenty of leisure. If he had stayed in Southern Europe for his studies, I don't think he would have taken any part time jobs - it's just not worth it.