Why wasn’t Kumayl ibn Ziyad and some other companions of Imam Ali (as) present on the day of Ashura or with Imam Hussain (as) at all?

Scholars have different opinions about that.

1- Some sources indicate he was imprisoned, so he could not go to Karbala.

2- Some clues indicate he was instructed by Imam Ali (a) to fulfill certain missions, so it was not an obligation for him to go to Karbala.

3- Some say he didn’t make it in time.

I believe the first one seems to be the most plausible reason.