evolutionarily speaking why is it so hard for humans to conceive?

There are two general methods organisms use to reproduce. 1) have as many children as possible and hope a few survive, (such as turtles and any organisms that have bunches of eggs), 2) have few children but heavily invest in that child doing all the parents can to insure the offspring's survivability. Obviously, humans (and most mammals) take the second choice. Now then, if a woman were to have many children, it would be more difficult for her to insure any one child's survival. Therefore, it might be that humans do not reproduce at high rates to be sure that the children who are born survive. Another reason that humans have one child at a time is because human children are totally dependent on their parents for many years. It takes a long time (relatively) for the complex brain to develop & become self sufficient. (For those of us whose children have bounced back, it might take a VERY long time for the child to become self-sufficient.)

Lastly, I might argue that it really is not difficult for a woman to get pregnant. One episode of sex can certainly do it.

Humans are one of just a few species where the females are always sexually receptive, rather than only going into heat when they're likely to conceive. Their times of ovulation are hidden, often even to themselves. They even maintain large breasts (which in other apes indicates sexual receptivity) all the time, despite the difficulties of carrying them around. In many cultures they'll also paint their faces in a manner to simulate sexual interest or excitement - adding rouge to the cheeks or red color to the lips, for instance. The most obvious benefit of all these characteristics would be to encourage monogamy in a male partner, which in turn would gain more resources and parenting effort for the children. The "downside", if you want to call it that, would be that we have to have sex more before conceiving. But that's basically the point.

Why bring evolution into it? That just invites speculation and "eat right for your type" ideas. Instead ask "Biologically, what are humans meant to eat?" Lactose intolerance is very common in adults and we could say that biologically we are not meant to consume milk as adults; but many people of european ancestry can. There is a long history of vegetarianism so people can survive quite well on such a diet but it is also quite easy to end up with specific deficiencies for nutrients that are not well provided for. This becomes more acute with a vegan diet. On the other hand Eskimo's have long managed on a meat and fish diet. So given that we can survive pretty well at both extremes it is most probable that we are omnivores and 'meant' to eat a mixed diet. Of course if you have specific food intolerances you adjust your diet accordingly.

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Most animals have a distinctive breeding season where the female remains in a state of high fertility for a short period and mating is almost guaranteed to produce pregnancy.

The world human population shows that we don't do so badly when it comes to reproduction.

I would also point out the roles of sex in pair bonding, maintaining families,and in making life worth living. So we do a lot of it, and for reasons that increase our fitness (in the strict darwinian sense) in other ways than the mere need to conceive.

To slow down the rate of human-caused ecodisasters.