Why?
First, 802.11n is still in Draft mode. The specifications have not been "ratified" into an accepted standard. Until is standard is "ratified", interoperability of the network devices is not guarantied.
I been experimenting with several 11n routers (DIR-615 and DIR-628) as well as other makers. I know that when you have "disconnect" problems in general, no matter what router "make" used, the problems tend to continue. You must eliminate:
-- Software interaction causes that forces the disconnects -- Norton "deauthenticates" wireless SSID if SSID not trusted.
-- Microwaves cause interference
-- Neighbour signal interference
-- Cordless "land-line" phones -- DECT 6.0 technology is "reported" WiFi friendly. Older 2.4 and 5.8 cordless phones can cause problem (I experienced this.)
-- Bluetooth signals -- These also can use the 2.4Ghz radio signals
-- Cordless keyboard / mice -- also on the 2.4Ghz radio
-- WAN port access problems -- Some broadband "modem" do not respond correctly if the WAN port line speed is set to Auto detect.
Also, do your "really" think that Linksys or D-Link really "make" the wireless radios? These companies "license" the radios from another manufacturer and tweak them to suit thier needs. Without going into greater details, it is possible that they both could use the same radios.
(Note: It has been posted elsewhere here that RALink has an updated wireless adapter drivers that can be used on several D-Link adapters. I have checked the INF files of these and there also several Linksys adapters mentioned.)