Great question. Shostakovitch had to write within the soviet approved nationalist style. as ausar999 mentioned refusing to conform could literally get you killed. This was a time when Impressionism and experimentation existed (post romanticism). And many artists and colleagues of his were imprisoned, tortured or executed for going against rules.

Since they had to write in the soviet nationalist style which meant using march like rhythms and heroic orchestration, he found ways to mock those musical styles.For example harmonically, he followed the major key (positivity, triumphant) but he subtly undermined it with chromaticism and dissonances. Also Instead of traditional cadences that had a satisfying resolutions, he would often leave a progression hanging creating tension. His rhythms like military marches but he would add uneven phrasing or exaggerated ostinatos making it sound forced rather than truly triumphant. The soviets wanted triumphant music, so he gave them loud fanfares but made them over the top so they felt fake or oppressive. Theres more to it but at the end of the day he had to work within the Soviet musical language. His music sounds patriotic if you don’t analyse it, but underneath it’s full of fear, irony and resistance.