It seems weird in my opinion, lots of European nations got their capital sieged and occupied once or twice. Austria did, France did, Prussia did, Russia did. There are probably more examples of this(I think Spain and Portugal also had their capitals occupied).
Ottoman Empire was a very old nation, founded in 1299 and fell in 1923. The only time I remember Kostantiniyye being occupied is after WWI by France and UK. Why did no other nation ever occupy it before this? Were their rivals(Russia, Austria, Persia etc.) too weak to occupy Ottoman capital or were there other reasons? It seems particulary weird Russia never did it, as it would reinforce their claim of being Third Rome, and also the fact that Constantinople and later Kostantiniyye was the center of Orthodox Christianity.
In 1853 the Ottoman military and all of its fleet was defeated by Russia in the Crimean war. Because Britain did not want to see Russia occupy the City, it intervened to save the Empire. So Kostantiniyye was saved from occupation by Britain, who, ironically, occupied it almost 70 years later.
Also, some years earlier, Muhammad 'Ali of Egypt would have certainly occupied the city if he had not met a Russian army, who was helping the Sultan, first.
The city was extremely defensible in previous centuries, but in the age of mechanized warfare the sultans had to rely on alliances to keep it protected.
Up to the late 1600s, early 1700s, Russia's focus was split between its ambition to be taken seriously as a European power, needing strategic access to the European markets and access to global trade. They had to contend on several fronts: the Baltics, the Black Sea, the east.
In that time period, Russia was simply not strong enough to beat the Ottomans alone. It tried to cultivate alliances with other European powers, but none came to fruition. Its southward ambition checked, it then focused on the Baltics, bringing it into war against Sweden.
In the 19th century, Russia was finally strong enough to threaten the Ottomans and wanted to gain access southwards, towards the Mediterranean. This time, it was viewed as a threat by other European powers. Thus, the Crimean War of the 19th century, which saw France and Britain ally to support Ottoman defense against Russia. Why was Russia so antagonized at this point? One reason was her growing strength. Another was her intervention in the War of Greek Independence. This is all part of the so-called Eastern Question, "Who shall control Constantinople?" which was and still is a very important strategic city.
So there were two distinct periods in the Russo-Ottoman conflict, each one deserving its own analysis.