Because it's a special case of a Func, and the guidelines say "don't use predicates":
LINQ Framework Design Guidelines[
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"Do use the new LINQ types "Func<>" and "Expression<>" instead of custom delegates and predicates, when defining new APIs."
Not all the Linq methods could use a Predicate since they can use more than one parameter of different types.
There is a good explanation here:
c# - Func vs. Action vs. Predicate - Stack Overflow[
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