question Why is Donald Trump so popular? was accepted, would question why Schulz is so popular despite failing 11th grade twice be accepted? How can I ask the question on why people support politician who failed 11th grade twice?
"Why is Donald Trump so popular?" asks an open, neutral, and "good faith" question. there are no assumptions or accusations in the question.
Which is different from the tone in your question. Specifically:
How people can consider a person like this for the office?
Does this fact show that democracy is failing in Germany?
This is a textbook example of a "rant in disguise". From What types of questions should I avoid asking?
To prevent your question from being flagged and possibly removed, avoid asking subjective questions where …:
- your question is just a rant in disguise: “______ sucks, am I right?”
It's pretty tricky to make a question in the form of "Why would anyone vote for person X in spite of trait Y?" not come off as a rant, so it's probably best to avoid those sort of questions altogether. A better question might be "What are the qualification requirements to run for office?", but I suspect that won't give the answer you're looking for.
A more general "Why is Martin Schulz so popular?" would also be okay.