That's because Pfizer vaccine is an mRNA vaccine that works differently with other vaccines. They instruct the cells to create a spike immune response to prevent the virus from latching on the cells. Antibodies aren't produced by vaccines, they are produced by the host and they will only be produced for a specific one if a specific virus have been introduced to the body albeit in a weak state.
There was a recent study with the AstraZeneca vaccine as well and it has been confirmed that this vaccine also shows a lower efficacy against the Delta strain. The most worrying aspect is that one dose of the AstraZeneca vaccine doesn't offer much protection against the mutant strain and this is going to create issues in countries such as India, which has relied on single dose regimen due to the shortage of vaccines. Double dose vaccination is going to take time and the mutant strain can cause a lot of damage by then.