Actually, within its historic context it is not difficult to understand, although one might find St. Isaac's to be Unorthodox in its iconography.
Beginning with Peter the Great there was a sincere interest in taking Russia out of the medieval period and into the modern world. As a result, various reforms were enacted including the prohibition on men having beards. One of the results of these reforms was schism within Russian Orthodoxy including a group called the Old Believers who resisted to the point of death.
These reforms continued until the very end of the Russian Empire in 1917. Peter the Great laid the foundations of his "Window on the West", St. Petersburg, and, at huge cost in money and lives, turned a swamp into a modern city. Although Moscow remained the capital of Russia, St. Petersburg, for all intents and purposes, was its functional capital. Both the magnificent Winter and Summer Palaces were constructed there. Catherine the Great imported architects from Italy and England to provide designs for buildings in the most contemporary styles in the eighteenth century. Thus, St. Petersburg is a world heritage monument for its superb examples of Baroque, Roccoco, and Neo-Classical architecture.
In the nineteenth century the tsars continued the momentum. They were generally sincerely religious. They did not see any reason that Russian Orthodoxy should remain bound to a distant and dead past. As with Rome, which adopted Renaissance architecture whole-heartedly in the sixteeenth century for its churches and St. Peter's Basilica, it was decided to adopt the same style for St. Isaac's Cathedral. If I recall correctly it was probably built during the reign of Alexander II.
The fact is that St. Isaac's is an exception which did not set a trend. The trend probably would have developed into the twentieth century had the Russian Empire continued. By contrast, Rome's experiment continued to develop and evolve, and does so still to this day.
I find just as reasonable to dislike the architectural and iconographical style of St. Isaac's Cathedral as I find it reasonable to dislike the architectural and artistic style of St. Peter's Basilica.