Because they were too hungry to care.
Austria issued Serbia a 10-part ultimatum in response to the role of Serbian and Slavic nationalists in the assassination of the Austrian archduke.
when Serbia refused to comply with Austria- Hungary's ultimatum
Austria was rightly angered. They issued an ultimatum to Serbia to right the wrongs committed, but the ultimatum was too much for just an assassination. Serbia didn't reply and Russia stepped in to help the small Slavic brother. That move turned the two-nation conflict into a war.
1. The ending of anti-Austrian agitation.2 . Punishment for those involved in the assassination of the archduke. 3. Enforcement of the ultimatum by Austrian officials.
Austria sent the ultimatum to Serbia on July 23rd 1914
Serbia
The assassination of Archduke Franz Ferdinand of Austria, by Gavrilo Princip on June 28, 1914. On July 23, 1914, Austria set an ultimatum for Serbia to leave Austria. This ultimatum would expire on July 28, 1914. When the Serbs would still not leave, Austria declared war on Serbia.
23rd July 1914
They gave Serbia an ultimatum which said that if they didn't stop trying to be independent then they would go to war with them, of course this then led to the assassination of Franz Ferdinand and the start of the first world war.
the germans
When Serbia responded to Austria-Hungary's ultimatum, they accepted all the conditions save one. Austria-Hungary immediately declared war on Serbia, which had always been their intention given the demeaning demands they had made.
Because the the Archduke Franze Ferdinand was the heir to the Austria-Hungary throne when him and his wife when assassinated by a Serbian Nationalist in Sarejevo. Austria Hungry Gave Serbia a ultimatum and when Serbia did not follow the ultimatum Austria-Hungry declared war with Serbia