Why? I think there are probably several reasons.
The quality and style of poetry did not accord with the mainstream style at that time. Wei, Jin, Southern and Northern Dynasties lasted for nearly 400 years, and Sui Dynasty experienced two generations of extinction. It was not until the Tang Dynasty that reunification was truly achieved.
Wei, Jin, Southern and Northern Dynasties were troubled times, and it was also the most serious time for the gate system. Due to the constant war, many literati and nobles are complacent and indifferent to the world, thus forming an "idyllic style of yearning for nature", which has become the mainstream of society.
This state had a great influence on the Tang Dynasty. Tao Yuanming and Xie Tiao were immortals in the minds of many people in the early Tang Dynasty.
At the same time, because parallel prose reached its peak in Wei and Jin Dynasties, this gorgeous but wordless style of writing also deeply influenced the style of writing in Tang Dynasty.
It was on this basis that Wang Bo, Li Bai and others were recognized by the society at that time.
Meng Haoran and Wang Wei's pastoral style has also been widely recognized, which is also because of this.
Du Fu's poems are mainly realistic. Although the materials are exquisite and neat, they are quite different from the mainstream style. Although this kind of poem was recognized by some writers at that time, its acceptance by the public was lagging behind, and naturally it would not be very popular.
In a well-connected society, few people pay attention to Du Fu, the bottom people. He wrote about the sufferings and life of the bottom people. Most of these people at the bottom are illiterate, and the educated people at the middle and upper levels can't understand the pain of ordinary people because of their different living environments, so naturally they can't sing.
Articles are emotional communication, and there is no way to have emotional voices. Naturally, they can't be recognized.
In the social state of the Tang Dynasty, the division of classes was quite obvious. People Wei and Meng Haoran generally don't have much good looks on Du Fu.
This also has something to do with his grandfather. Du Fu's grandfather, Du Fu, was a famous poet in the early Tang Dynasty.
However, he was arrogant and mean, and his reputation was extremely bad when he was an official in the DPRK. During the period of Wu Zetian, he colluded with Wu Zetian's favorite to frame Li's children. Fortunately, he died early, otherwise he would definitely be executed by Li Longji Song Like Wenzhi.
With such a family background, Du Fu is talented, but Li Longji certainly won't reuse him, just don't suppress him.
This should also be the reason why Du Fu failed to get an official position in Chang 'an for ten years.
To sum up, it is understandable that Du Fu's poems were not recognized in his time.
In the late Tang Dynasty, people paid more attention to the individual, because of the development, the social structure also changed a lot, and the literati had more requirements for the rhythm and antithesis of poetry. Du Fu's poems just met the needs of the times, so they were quickly accepted by the public. After being widely circulated, so far.