Lots of variables for a lot of reasons. Could be medical, too many kids, etc…
I know the biggest killer of sexual desire in a woman for her husband is to be made to feel as not special, to not be valued, to not feel as a equal partner, to be lied to , to be cheated on, or even to be verbally or emotionally abused. These things KILL the woman’s desire to open herself up to the man. A woman wants to be appreciated for all that she sacrifices. A woman needs this!!! A woman want to feel safe in all areas of the relationship. She may not expect you to sacrifice, but if you don’t sacrifice, it will be taken as she doesn’t matter enough to you for you do so. That will close her down!!! Feel free to read between the lines here.
The best way to keep a woman open and wanting to open herself up to you and give herself to you is to remember that it is not just sex. It is her giving her all to you. Treat her and everything that makes her, her, with respect and like she matters, like she is the most important thing in your life, sacrifice for her, take care of her, you probably won’t have a problem with the frequency of sex. There is nothing sexier than a man who is in tune with his wife, loves her through all of her faults, supports her in every way, and gives her all of himself.