Isn't the Pope entitled to excommuicate rulers and issue Papal bulls empowering/bestowing the crown upon a rival claimant?
Entitled to excommuniate yes. Power to to say "you're the king" no.
Pope is giving legitimacy, no reality of power : that's a job for nobility.
A pope trying this sort of things would have to fear a "Agniani's slap" far earlier than OTL : not only from the guy concerned but also from the kings that supported papcacy '"What the heck? He can't remove king like that, if i support that, it could backfire".
Because the king was the pillar of feudal society, HE gave legitimacy to the power of nobility and it was why great nobles never tried seriously to overthrow him. A king supporting a king-maker pope would send this message to ambitious :
"Are you tired with your king rule? Did you always wanted to bear a better crown? Why hesitate, when the pope can remove your rival and give you enough power to be satisfied with! Call the 555-9225 and if you answer in the ten days, you'll recive a wonderful relic of Saint Anselm!"