As well as the above reasons, Google has a support article on QS:
[h=3]"How we calculate Quality Score[/h] Every time someone does a search that triggers your ad, we calculate a Quality Score. To calculate this Quality Score, we look at a number of different things related to your account. By improving the following factors you can help improve your Quality Score:
- Your keyword's past click-through rate (CTR): How often that keyword led to clicks on your ad
- Your Display URL's past CTR: How often you received clicks with your Display URL
- Your account history: The overall CTR of all the ads and keywords in your account
- The quality of your landing page: How relevant, transparent and easy-to-navigate your page is
- Your keyword/ad relevance: How relevant your keyword is to your ads
- Your keyword/search relevance: How relevant your keyword is to what a customer searches for
- Geographic performance: How successful your account has been in the regions that you're targeting
- Your ad's performance on a site: How well your ad's been doing on this and similar sites (if you're targeting the Display Network)
- Your targeted devices: How well your ads have been performing on different types of devices, such as desktops/laptops, mobile devices and tablets - you get different Quality Scores for different types of devices"
Source: https://support.google.com/adwords/answer/2454010