Question:

What causes a low quality score?

Answer:

1) If your Ads, landing pages, and site doesn't match very well to the keywords, then that will give a low quality score.

2) Too many different kinds of keywords in one ad group and too many different
kinds of ad groups pointing to the same landing page can contribute to a
low QS.

3) If your site doesn't have much content, or Google's bot can't easily find it

4) If you're bidding on keywords that most advertisers have difficulty
achieving relevance on, then that keyword has been trashed by other
advertisers and it will negatively affect your QS.