My guess is so that you had a realistic chance to checkmate him. If you had to checkmate the "King with Queen powers, it'd be a lot harder, especially towards the end, since the "King WQP" could probably capture most pieces that were checking him, so most games would probably end in a draw.
If the king is so important, why did they only make him move one square any direction?

what I think is happening is that this conception confuses value with importance, which are not the same things here: the king is so important that he does not have material value, or he has infinite value
importance vs ability/power
The king is important, but if you give him too much power, it becomes nearly impossible to checkmate him. In pretty much all endgames, checkmate is impossible. For example, try this:
Black is up by a whole queen, but there's absolutely no way black can even hope to come close to checkmate!

It would be fun to figure out the mating position where the attacking side needs the least material...
Can anybody beat this?
It would be fun to figure out the mating position where the attacking side needs the least material...
Can anybody beat this?
lol, it's black's turn but white is in check. this can get very crazy. they should make a new variant where the king has these super powers.
It would be fun to figure out the mating position where the attacking side needs the least material...
Can anybody beat this?
Ooh, I can!

It would be fun to figure out the mating position where the attacking side needs the least material...
Can anybody beat this?
Ooh, I can!
Sorry, No, your starting position is illegal. Both Kings are in check :(

A male lion is considered king to, he doesn't do a lot hunting is done by the female lions. Like people who are in control, they don't do the heavy work i think it makes sense a bit. People who rule have other people to do most of their unpleasant work, and so in chess king is a bit lazy..
It's the way how we man would like to live, having our queens run for us :))

Given that the origins of Chess lie within the parallels of ancient organized battle, it would be wise to keep the absolute leader far from harm and let him delegate responsibility to his generals who willingly put themselves at risk for their leader. This gives the leader little incentive to be battle-worthy himself. Just a thought.
Looking at it a little differently => I'm sure most grown adults in this forum would willingly step into the ring with Adolf Hitler and engage in hand-to-hand combat ... and possibly beat the tar out of him.
Though that didn't make him and his million-drone army any less dangerous.

Because he is "the important one, the chosen one". He just sits there , doing as little as possible, making all the others do all the work to keep him safe- like a real king! It's a wonder he moves at all!

In ancient chess ( It was not called chess then..) the king and queen both had very little mobility. One Square. And the pawns could not promote eighter.
As Chess became "modern" they changed a bit in pawn movement.. en passant, 2 squares option in first move and promotion to any officer.. ( except King I guess ?? LOL) and the Queen turned into a "monster" with the Movement of both Bishop and Rook at the same time. This was done to make the game universal (all rules the same worldwide) and more exciting at the same time.
It is at the same time a war game, where you move around with your army and you can sacrifice parts of it too, with the only goal to capture enemy king, nothing else matters!! By the same time it is a gentlemans game, where you usually quit before that when you see it is hopeless, and also the King cannot be "killed" he just can be captured so he cannot move out of chess next move. (No escape)
Since King cannot leave the table, be taken away or sacrificed it is impossible to give him any kind of value. In most games you hide him away as best you can, and just in the later part of game he enter battleground with more or less success. So, it is like on sea, the captain goes down with his ship!
Just a general question. But it makes sense. Since the queen can move tons of places, and most games where one loses the queen ends up resigning next move, it seems to me people treat the queen more valuable than the king. So then why did they only let the king move one square? Doesn't that make him less important, although the object of the game is to not "give him up"?