Have you let her know this?
Many women feel that to initiate sex would be "unwomanly", only sluts do that. Also, many women feel that to do so might infringe on the man"s territory. He's supposed to be the dominant one. There certainly is some truth in the second explanation. Some men would find such an act to be threatening to their "status". Personally, I think that is nonsense. If a man is threaten by that, then he ain't much of a man.
I'd suggest you talk to her about that. You may find that she may want to be a little more assertive sexually.
Historically, between my wife and I, I initiated sex nearly all of the time, but there were exceptions. However, since the birth of our firstborn, she has been much, much more assertive, even to the point of randomly coming up to me, undoing my pants, and blowing me. At first, it really caught me off guard. But it has become a fairly routine thing for us. And she isn't really interested in any reciprocation when she does it. (I find that a little frustrating.)