Why is hard for pandas to breed in captivity?

There are many reasons.

For one, female pandas are in heat only 72 hours a year. This is a very short window, and if it is missed, the researchers need to wait an entire year before that female can breed again. For this reason, their hormone levels need to be monitored every day.

Being in captivity also has a negative affect on Panda libido. This is true for many animals, not just Giant Pandas. If the female doesn't feel the environment can support her cub, she won't bred. Any factors that may bring her stress can also have a negative impact on her willingness to reproduce.

Also, male Pandas have very small penises. If the positioning of the breeding pair isn't perfect, the male will fail to impregnate the female.

Interaction between captive Pandas can also be dangerous. If the male and female are not interested in each other (and they are often not) they may show aggression.

For all these reasons, Pandas are usually artificially inseminated. This has been quite successful.