Here’s Why this happens:

Go Away, I Only Want Mummy – Or Daddy. When Your Child Prefers One Parent

Why Dads Don’t Need Bottles To Bond With Their Babies…

Why Won’t My Child Settle for Dad at Night?

Why Does My Child Shun Me In Favour Of Their Mum?

Carry Me Daddy!

And how you can work through it gently.

You have to scaffold it. Start with having dad just there with you while you do bedtime.
Then after a good week or more of that, have him do part of the bedtime routine. Eg Nappy change and dressing them.

Stick with that for a few days then slowly increase Dad’s help. He does the bath or the book. But you’re still there too. You’re tag teaming.

Over at least a month increase to Dad starting cuddling/rocking/patting to sleep then swapping with you (and staying there) if you sing songs try singing them together. 

Then the next month you work on extending the amount of time dad cuddles to sleep.

Only once baby is happy and feels safe with dad doing the bedtime do you start ducking out for a couple of minutes at a time. Again slowly increase this time over a few weeks.