There are different factors:
There's a lot of women and children that do come, but they don't come on boats like that through the channel, well because it's pretty dangerous, they come through different legal means and there's usually ways set up by European governments for legal immigration for families that are fleeing war or displaced somewhere to be brought to the country legally, since women and children are seen as vulnerable and a priority, therefore more help. While there's no such thing for single men, which leaves them only very few options like taking a dangerous boat ride.
There's also the factor that because men are better equipped to take the hard journey than women and children who usually stay in other countries, and wait for the men to make family reunion applications for them once they arrive in their destination country. Also the fact that some of these men are fleeing military conscriptions in their countries, while women don't have to worry about conscription and getting drafted to be blown up in some tanks.
As for why this is happening, while I have a bunch of opinions that I don't think would be very popular on this place and I can't be bothered going into too much detail about them anyway. However the liberal "lets welcome everybody" mentality, while it might seem like it's coming from a good place and maybe it is, is not very wise. And the right mentality of "illegal immigration is illegal therefore it's always bad, go back to where you came from" is quite hateful and ignorant. Both sides get it wrong on immigration and both come from a place of ignorance.