Why de-power Black Bolt.

I mean, in a few years we've been from someone capable of matter manipulation, leveling cities with one word, faster than the lightning reflexes, punches at least as powerful as the Thing's ones to someone who can easily be defeated by a bunch of Skrulls or be turned to stone by a second class villain like the Basilisk ?

Why make such drastic changes. He is after all the king of the Inhumans. What good came out of this constant de-powering.

Because he was overpowered and it was hard to write a story for him. You have a guy who can kill the most powerful villain with a whisper .. Now how do you make the reader believe that he's in any real kind of danger?

U dont know what story arc he is in but I would agree if he were used more often. BB doesnt seem to mess with everyone else ( except for realm of kings). He is one of the most powerful heroes on the planet though