Why is Sir Walter Scott considered a great writer of Romantic verse tales?
First of all, we must understand that Scott was a "Romanticist," with a capital "R." Romanticism was a literary movement of the late 18th century which heavily emphasized the importance of nature and man's emotions and imagination in conjunction with, and in response to, the natural world. Be aware, too, that "Romanticism" does not always mean nice or pleasant. The natural world is often brutal and at odds with man.
Scott was (arguably) the first true international literary celebrity, finding readership and fame not only in Europe but also in North American and Australia. His works, such as Ivanhoe, the tale of a knight on a quest, remain popular to this day and were wildly popular in Scott's time. Other popular works include Waverly, The Black Dwarf, and Rob Roy:
The reason that Scott is considered a "great writer of Romantic verse" is because he embodies the tenets of Romanticism outlined above: that is, nature and emotion, and man's actions and responses to the natural world. Here is are two examples from Ivanhoe:
“Those who remarked in the countenance of this young hero a dissolute audacity mingled with extreme haughtiness ... could not yet deny to his countenance that sort of comeliness which belongs to an open set of features, well formed by nature, modeled by art to the usual rules of courtesy, yet so far frank and honest, that they seemed as if they disclaimed to conceal the natural working of the soul.”
"Meantime the clang of the bows and the shouts of the combatants mixed fearfully with the sound of the trumpets, and drowned the groans of those who fell, and lay rolling defenceless beneath the feet of the horses. The splendid armour of the combatants was now defaced with dust and blood, and gave way at every stroke of the sword and battle-axe. The gay plumage, shorn from the crests, drifted upon the breeze like snowflakes. All that was beautiful in the martial array had disappeared, and what was now visible was only calculated to awaken terror or compassion.”