why is it that during the ghaibat of Imam e zaman Eid namaz becomes mustahab but still khums of sahme imam still remains obligatory?

Walaikumussalam
Because khums is wajib fir the reason that it is clearly mentioned in the quran as an obligatory act
And namaz e Eid it must be lead by Imam Masoom and imam is in the ghaibat so it is mustahab
Wassalam