There are a few big things to think about in the time frame from the fall of Constantinople in 1453 and the final collapse of the Ottoman Empire in 1918. First off, despite the common perception that the Ottomans were continuously declining, for much of that period we can talk as much about the conquest and colonisation of Constantinople as the conquest and colonisation of Lisbon or Madrid. The Ottomans were simply too powerful to be conquered and colonised (much like other established states in Subsaharan Africa and all over Asia). Remember than in 1683, the Ottoman conquest of Vienna was a real threat. Ottoman armies were threatening the heart of Austria, not Austrian armies threatening Constantinople. Even after their defeat at Vienna, and the subsequent Habsburg conquest of Hungary, the Ottomans remained a significant power, able to hold their own against the Austrians and Russians.

However let us look forward to the mid to late 19th century, when new technologies were enabling European forces to make significant inroads into Africa and China, and the Ottomans were considered the “Sick Man of Europe”. At this point, the question of which power would control Constantinople and it’s inherent strategic value affected policy all over Europe. Commonly known as the Eastern Question, the issue of how to handle the weakening Ottomans was a major concern for European governments. While the Russians would’ve been more than happy to conquer Constantinople, many other European powers were less than enthused by the idea of a Russian controlled Bosporus and a strong Russian presence in the Mediterranean. Britain in particular saw the preservation of the Ottomans as in their strategic interest, and the Royal Navy was a major buttress for the Ottomans. In fact, the mid-19th century saw both France and Britain fighting Russia alongside the Ottomans in the Crimean War. Similarly, the Austrians saw the preservation of the Ottomans as keeping the Russians from dominating the Balkans, though this didn’t stop them from plotting partitions of the Ottoman Empire from time to time. This state of affairs would persist into WWI.

None of this is to say that the Ottomans were passive during all this. Alas, I am no expert on Ottoman history in this period and will leave it to others to discuss internal Ottoman efforts to maintain their empire. However, suffice it to say that for most of its existence, the Ottomans were either too powerful to be simply conquered, or an Ottoman collapse would be disadvantageous to major European powers, thus opposed by them.

I hope this serves to answer your question. Please feel free to ask any follow ups.