It’s because the features were implemented by vendors before the specification reached its final release stage.
The vendor prefixes ensure that there are no clashes with changing functionality etc.
Originally, the point of vendor prefixes was to allow browser makers
to start supporting experimental CSS declarations.Let’s say a W3C working group is discussing a grid declaration (which,
incidentally, wouldn’t be such a bad idea). Let’s furthermore say that
some people create a draft specification, but others disagree with
some of the details. As we know, this process may take ages.Let’s furthermore say that Microsoft as an experiment decides to
implement the proposed grid. At this point in time, Microsoft cannot
be certain that the specification will not change. Therefore, instead
of adding grid to its CSS, it adds -ms-grid.The vendor prefix kind of says “this is the Microsoft interpretation
of an ongoing proposal.” Thus, if the final definition of grid is
different, Microsoft can add a new CSS property grid without breaking
pages that depend on -ms-grid
Source.