Surely a quick Google can tell you that?
The EHRC published a report
in 2017 talking about the reasons for the gender pay gap. Many other think tanks, organisations and Nobel prize winners have also researched it too.
There is no one single reason, but there are a lot of factors as to why there are more women than men on low wages. For example, women tend to need work which fits around other obligations such as parenting, caring, etc, and a lot of retail, hospitality and shift based roles are both flexible, and low paid. Women often need to work part time and part time worm is statistically paid less than full time work. This is because women shoulder a disproportionate burden of unpaid care work in our society. There are many many studies on this.
On top of that, what is seen as womens work is consistently undervalued socially, compared to what is considered mens work. Occupations with a high frequency of women employees are underpaid when compared to male counterparts. This is an interesting study on the phenomenon: academic.oup.com/sf/article-abstract/88/2/865/2235342
Also, women have often been discouraged at best or actively blocked at worst from some roles. My MIL was a doctor and she remembered how hard it was to get ahead, and how few women there were, when she first started out in medical school in the 70's. I know of women now in the tech industy experiencing the same challenges she did, moving forward in male dominated companies. There are also more men than women in senior roles. Girls looking at the world of work have fewer role models and mentors when it comes to higher paid work.
Maternity leave obviously plays a significant role as well, in limiting women's earning ability, and putting employers off hiring, developing and promoting women. It shouldn't but it still does, sadly. Pregnant then Screwed will have lots of info about that.