If firearms designed 200+ years before are still in use by the military of any Country (guerrilla forces are going to face different priorities) it would mean at least one of the following (but not necessarily all):
A - general tech development has slowed down.
This may happen for economic reasons. R&D of new weapon systems is extremely expensive. That would prove quite unpopular if basic health care cannot be kept because of the ongoing economic conjuncture. Another combined reason may be lack of pressing need. One thing is if you fear any moment the enemy is going to land on your shores. Another if tensions with other nations are somewhat managed in a non violent way.
B - they are still somewhat effective.
Which means that development of personal armor has not progressed much. Maybe for reason A. Maybe effective armor is just too expensive to be given to the average grunt.
C - They are just a part of the arsenal.
While EM rifles and anti-droid weaponry is an established technology and nanotech is the cutting edge the good old firearms of the past are more than adequate to keep in check the swarms of infantry. After all grunts are much less expensive to field in 2191
D - Inertia.
As pointed out in the other answers when you and your allies have large stocks of weapons and ammo it becomes difficult to justify to switch over to a complete new system. Especially if point B is valid.
Furthermore: any firearm we have in use now is any good if the user is capable of detecting and aiming correctly at the target. So while a SCAR 17 will still be an awesome weapon in 200 years the big upgrade would be the aiming system, battle awareness, droids integration and so on.