Why 11.0592mhz crystal is used with 8051?

Answer 1

In some systems the CPU and bus timing is a limit to the highest and sometimes the lowest CPU clock frequency, so the clock needs to be stable enough to guarantee timing requirements are met according to the design and the specification.

Answer 2

Because that was what was selected by the designers to make it easy to set the UART BAUD rate selections.

Modern 8051 replacements run faster.