Possibly because they are DAO, I don't know. While I've seen ad copy stating 'low bore axis', they certainly don't appear to have a low bore axis and in fact to my eyeball, they appear to have a fairly tall bore axis comparable to SA XD/XDM guns and Sigs.
Now, while Smith & Wesson states their M&P line is 'double action', they are most certainly NOT double action guns and are as single action as any handgun ever made. Likewise with Glock--'safe action' is something of a hybrid, adding some striker cocking on trigger press. However, while they are not single action, they are only somewhat double action.
I don't know if this is the reason they are less popular than other brands, but my feeling is that full DAO guns are going to be limited in application, and especially popular for pocket and possibly BUG use, less so as a primary full size. Probably only somewhat more popular are the DA/SA guns (e.g. Sig).
Now, not to take away from your personal preferences, but I find the FN pistols (and the FNX's I've seen) to be particularly butt-ugly guns and a bit on the pricey side for butt-ugly guns. So, if aesthetics weren't important to me I'd likely go with a Glock--to get less of a DA feel, far lower bore axis and an unlimited supply of aftermarket crap. It could be my view is shared by more folks than those who like the FNs. I don't know.
It could also be that they are not as well-distributed in the states as the other brands.
I wouldn't let any of this speculation and opinion sway you from trying to find one to shoot, however. I'm confident they make a high quality firearm and you may actually want a DAO pistol. In my case, I actually wanted a small carry weapon and DAO was very acceptable and even attractive. I chose a Kahr to fit the bill. For a full-size carry or night-stand gun, however, I simply wouldn't choose the FN platform for all the reasons mentioned.