R' Chaim, the brother of the Maharal, tells us that the letters י,ה,ו are spiritually higher letters than the rest of the alef-beis since these are the letters that comprise Hashem’s name י-ה-ו-ה.
Similarly, R' Chaim teaches, the letters ב,ש,כ,מ,ל,ו are higher spiritually than the other letters of the alef-beis. Why is this the case? Because it is only these letters that are associated Hashem’s name, י-ה-ו-ה in that they are used as its prefix. To illustrate: בה', שה', כה', מה', לה', וה'.
It is because of this that these letters were chosen for ol malchus shamayim as ב,ש,כ,מ,ל,ו are the first letters of ברוך שם כבוד מלכותו לעולם.