The real reason

The Krovetz LePurim  is  on every Bracha
of the Shmonei Esrei except on the Bracha
“Es Tzemach”.
 Some say, the reason for
this is, "All the wars were fought by Malchus
Bais Dovid except for Milchemes Amalek
which were fought by the grandchildren
of Rochel. Mordechai was Mesaken for
Shaul Hamelech's war on Agog.
Therefore we don’t mention the Nes
when saying Es Tzemach Dovid."

The truth is, Krovetz, were written not only 
for Purim.
There are Krovetz for Hoshana Rabah,
17th Tamuz , 10th Teves and 9thAv.
They all have no Krovetz  on Es Tzemach.

The Paytan for Krovetz was R, Elazar b Hakalir.
In his Piyutim he generally follows the
Yerushalmi over  Bavli.

According to the Yerushalmi , the
Bracha of Es Tzemach and Vilurushalayim
Ircha, are combined into one Bracha.

Hence, no Krovetz for Es Tzemach.