Why search engine penalize a website?

A Google penalty is the negative impact of the search engine on the website ranking. It may be a by-product of the algorithm update or if the website has intentionally used black-hat techniques.
The reasons why your website may be penalized-
1. Buying links to increase the number of backlinks coming to your site.
2. Excessive number of reciprocal links.
3. High quantity of duplicate linksInternal 404 error.
4. Black hat techniques like- keyword stuffing, hidden links, hidden content.
5. Using the links in the footer section to pass excessive link juice.
6. Missing the sitemap data.
7. Broken external links.
8. Neglecting Hreflang- ‘Hreflang’ is designed to notify Google that you have intentionally published duplicate content for different languages or localities.
9. Website Timing out again and again.
10. Affiliate links all over the website.
11. Overusing the Meta tags in the website.
12. The slow loading speed of the website.
13. Hacked content.
14. Link to suspicious sites.
15. Excessive blocking of the site using the robots.txt file.
16.  Over optimizing of the website.
17. Too many outbound links.
18. Error codes in website- apart from 404 error , there are other errors in the like 302(temporary redirection), 500 errors, etc can also  lead to penalty.
19. Poor mobile websites.
20. Domain has a bad reputation.
21. Using link farm i.e. multiple links at one place or link spamming.
All these error may not be the direct reason to penalize the website but play a part in the process.