When you boil it down most women are attracted to men who would make a good father. This usually includes some level of physical strength, healthy appearance, social competency, and solid character to pass onto children. Men who complain about not getting laid are obviously missing a few or all of these traits. In reality, not every man is cut out to be a parent, and so not every man is desired by women.
Of course, the above isn't a strict rule and many men get laid for a variety of reasons and in a variety of contexts. But at a bare minimum if someone is attractive there is at least one trait present which would help their offspring procreate themselves, be it physical appearance, strength, confidence, or character. If someone lacks all four of these traits, be it man or woman, they're going to have a hard time.
This is where the evolutionary angle of sexual selection comes into play. Attraction is generally going to be different for males versus females of almost every mammalian species. For males, 'make good children and don't die in childbirth' is a core reproductive drive. For females, who bear the burden of childbirth, it's more like 'make good children and protect me while I gestate and then provide for and protect me and my children while I rear them'.
'Don't die in childbirth' ends up underlying some elements of physical attraction - youth, large hip-to-waist ratio, indicators of health like skin and hair and nails, etc. On a sociological note, a lot of the peacocking that women do is targeted those elements, even if it's subconscious. Most make-up either emphasizes or mimics that characteristics of youth by presenting smooth, glowing skin and healthy shiny bouncy hair. Even manicures end up suggesting healthy nails, which is an indicator of overall health. And most of the clothing that men consider to be sexy are outfits that emphasize or exaggerate hips, waist, and breasts - indicators of a woman's ability to bear a child to term without dying.
'Provide and protect' similarly show sup in the selections that women tend to make. Stability, maturity, and social success (not explicitly financial success) are big ones, as they strongly correlate to the ability to provide. Basic indicators of health are of course cleanliness, grooming, etc. Character ends up playing a larger selection role for most women, because the ability to provide and protect isn't as closely tied to youth or to physical attractiveness in males. A middle-aged man with a receding hairline and an oft-broken nose isn't necessarily off the table. Virility doesn't necessarily reduce in men until they're at an age that would historically have suggested that they're no longer in a position to provide and protect anyway. And that broken nose and those scars can frequently indicate that a man is willing and able to stand up and fight when necessary.
That said... sexual selection isn't exclusively biological in nature. Some of it probably is innate based on years of evolution, particularly those elements focused on health indicators. But a lot of it is going to be social - especially when we're talking about the keys that women are looking for to indicate success and stability. Women are no longer relegated to the home, so the need for a man to be a breadwinner is significantly reduced. A man can be a good provider and protector by being willing and able to help with the housework and to care for children. The criteria for provider/protector/partner is in flux.
