"There are a lot of modern and working faster microcontrollers"

There are very many criteria in the choise of an MCU - speed is only one of them.
Not every project requires the very fastet; eg, my current project uses a Triscend at 24MHz - that's only just over half the chip's top speed!

The 8052 is a mature architecture, with mature support, lots of experienced developers, and supported by many different manufacturers - these can be significant factors in the success of a project!