Why is Luoyang also called "the capital" in Tang poetry? For example, "Only the peony is the true national color, and the capital moves when it blooms."

Because Luoyang was also the capital of the Tang Dynasty. Wu Zetian's dictatorship was based in Luoyang. In September 684, he changed the eastern capital to the God Capital, and sent officials to stay in Chang 'an. In 69, Wu Zetian put down the rebellion in Xu Jingye, abolished Tang Ruizong, succeeded the emperor, changed the title to Zhou, which was called Wu Zhou in history, made Luoyang its capital, changed Luoyang into the capital of gods, and made Li Dan an heir. Wu Zetian became the only female emperor in China history. From 684 to 76, Luoyang was the de facto capital for 22 years. In April 883, Tang Jun recovered Chang 'an and then Luoyang. In the first month of 94, Tang Zhaozong moved the capital to Luoyang, and died in the Tang Dynasty in April of 97. Luoyang was the capital for three years. As for the peony in the poem, it is also related to Wu Zetian. In history, there was a legend that Wu Zetian demoted peony to Luoyang and described Wu Zetian as a tyrant. Actually, she is a very emotional person, especially like peony. According to Shu Yuanyu's Preface to Peony, "Xihe River, the hometown of Tianhou, has many fragrant houses, and there are peonies under it. Its flowers are unique, and Tianhou (then Tian) sighs the lack of gardens, so it is transplanted because of fate." This shows that there are many peonies in Wu Zetian's hometown for a long time, and the varieties are "unique", which has surpassed many varieties in Beijing. Therefore, the rare varieties in her hometown were moved to the capital. That's why there is the poem above.