I think a lot of it is demand. Tech like computing power has rapidly advanced because the demand has increased dramatically, especially the electronic warfare field. Right now a lot of the advancement is geared towards air and seapower, esp unmanned, because it is the kind of war we're fighting. I think it'd take some sort of mass engagement of modern infantry, or a working set (and proven effectiveness) of advanced personal armor ( like the Talos system they were working on) to spur innovation in small arms. I think as long as what we've got is effective against the current opponents we'll probably stick with it.
