This might be a totally naive question, but i am interested to know the particular reasons. Why was PDDL developed in the first place? Why could we not do the reasoning using First Order Logic?
Using a more specific language to express your problem makes it possible to apply more specific algorithms to solve them.
From a theoretic point of view FOL is undecidable while most flavors of PDDL are still decidable, because PDDL can only express planning problems. And e.g. classical planning with parameterized actions is "only" like EXPSPACE-complete.
Of course an EXPSPACE-complete problem expressed in more general/expressive FOL is still solvable in EXPSPACE, if you know about it. But how hard is it to come up with a general FOL solver that guarantees to solve all problems that are in EXPSPACE using only exponential space?
On the practical side, expressing a planning problem using a language designed for modeling planning problems is far more convenient than writing it down in FOL.
Wouldn't you prefer to write C++ instead of Assembler? Even though everything you can write in C++ can be expressed in Assembler.