Could be anything from legit reasons to the colour of your skin or what you put in the 'equality' form. Even if we knew everything about you, a company's vetting process is mysterious and individual, so it's impossible to say. Even if you ask for feedback, all you'll get is the exact line "other candidates had more relevant skills and experience".
My tip is to not bother applying, or at least spend little time tailoring your cover letter or CV. This is for entry-level jobs, anyway. When you spend all that time tailoring and get 200 more rejections, that's a lot of time wasted and your emotions hit rock bottom, even with lethal outcomes. However, you'll be much happier just uploading your CV to sites where employers search for CVs and spending all your free time doing whatever fulfills you. In my case, I got a slew of jobs as soon as I stopped trying and used all my free time being happy rather than sharing it between 'happy stuff' and 'boring, boring jobsearching'.
My first recommendation would always to be apply for jobs, with a good cover letter and CV at least until you've racked up a couple-hundred rejections. Then it becomes clear you need to change something. Unfortunately, the tweaking of CVs by third parties is a vicious circle, or more accurately a road that keeps forking yet always leads to nowhere. Third parties can't even agree if you speak in first or third person in CVs, 'experts' from all sectors and levels of experience have differing opinions on areas.