Why did some Russian musicians criticize Pyotr Ilyich Tchaikovsky during his lifetime?

During his lifetime, Pyotr Ilyich Tchaikovsky faced criticism from certain Russian musicians primarily because they viewed his musical style as insufficiently nationalistic.1

This friction stemmed largely from his position as a "Westernizer" with formal conservatory training, which contrasted with the goals of a rival group of composers known as The Five. This group sought to establish a distinct school of Russian music that was free from the influence of Western European forms, such as Italian opera and German lieder. While Tchaikovsky did incorporate Russian folk music into his compositions, his work also drew heavily upon French, Italian, and German musical traditions, leading some contemporaries to perceive his approach as lacking the necessary nationalistic focus favored by his rivals.21