[Mesorah] Yaaleh ViYavo

[D&E-H Bannett]

The explanation I remember for the omission of melekh is 
that we have just mentioned malkhut bet David in the 
preceding paragraph and mentioning malkhut Hashem just after 
would put them both on the same level.  I also remember 
having seen siddurim where the word mekekh is added on Rosh 
Hashana. R' Amram Gaon has the word melekh in birkat hamazon 
but R' Sa'adia Gaon and other siddurei ha-geonim omit our 
last line ki El ...  completely so have no problem of adding 
or omitting any particular word in a non existing line.

David