One of the reason was Ottomans treat subjects too good.never try to assimilate them.Real problem was population. World war 1 the Turks number about 10 million. The Russians over a hundred million while the english and french are 40/50 million each. I think even in our relative backward state going into world one, had been just 20 million, let alone 40 or a 100 millions things would have been very different.
Also the Turks faced hundreds of years of coalitions which ultimately has the effect of grinding you down. Imagine if the Hapsburgs had to fight off a hundred Muslim coalitions, where not only the Ottomans came, but also the Safavids, Mughuls and countless others etc.
One thing you have to give the christians, they knew how to come as a gang, muslims on the other side never really gave to europe the way they gave it to the Ottomans.
Compare to what the Christian (especially Spanish) done to Muslims and native American.They forced everyone to change religion or be wiped out.
Ottomans had 40 millions population in 1840s but only 7or 8 millions Türkçe speakers and loyalist.