At the time of the Ottoman invasion you had the Byzantines, another Bulgarian empire, et cetera. Mainly quite strong nations- so why did they fall so easily? A follow up question would be why the Byzantine empire never named itself Byzantine and instead became Greece? And why did they not reclaim Istanbul/Constantinople?
As a (one more) follow up question, there is (probably unofficial) term "orientalization" about development of russian army in XV-XVIth cent. (mostly, switch from spearmen to light sabre cavalry as an army basis.) It is said that russian army became quite similar to an ottomans' one. Was there similar process in other countries?
See [russian]: В. В. Пенской. Ещё раз об "ориентализации" в сер. XV - первой пол. XVI вв.