They don't have a simple-to-use alternative that has better piercing capabilities and is easier to conceal....
I'd like to have crossbows too, but I can see how they'd rather focus on making the core weapons feel good and fun to play with..... I'm actually mostly pro variety -- but I won't complain too much.
But that is what makes the crossbow unique while a bow is used because it is cheaper(compared to a crossbow), has generally longer range. I disagree it would take away from the bow in any way, its the playe's decision on what he will use and besides if they are in you just have to find clever ways to balance them.
For example - Crossbows will of course have rediculous loading times if taken from a more realistic approach and is only suited for assasination type missions or a first strike type of assault. Also the weapon and the ammunition are would be very rare cosrtly to find and use.
I won't be sad if crossbows are missing but I will be excited if they are confirmed.