WhatsApp application launched in 2009 has test cases developed in the sameyear for testing. When these test cases were used to test the application in2011, no issues were identified while executing the test case. What could beprobable reason for not able to identify bugs?

As per the pesticide paradox principal, If the same tests are repeated over
and over again, eventually the same set of test cases will no longer find any  new bugs. As over the years, application is updated and upgraded so executing  the same test cases will be redundant and executing them will not lead to  identification of any issues.