Chicken Soup Question

Friday i made chicken soup for the first time using broth. In the past i've always been a purist and boiled chicken parts for hours on end to get the real flavor. This time i decided to cheat. Only ingredients i used were carrots, 3 stalks of celery,black pepper, broth cans, 1/4 of an onion and 2 bnls chicken breasts (originally frozen). Everything came out fine after simmering for about 1 1/2 hours which gave the soup plenty of time to let the flavors blend.

The only issue i had was that the chicken seemed dry. I baked the chicken breasts for 15 minutes and then chunked them up into the soup so they could finish cooking. Not sure if it's especially a good idea to toss in raw chicken when people keep on sampling the pot for flavor every couple minutes! ;)

My question is, why was the chicken dry? Is it because i baked it for a few minutes before tossing in the pot? Ot would it be because i used cheap frozen chicken in the first place?

In the past i traditionally use bone in chicken which of course adds to the flavor and is obviously more moist. I've never used bnls chicken before. I've baked my fresh bone in chicken and then tossed it into the pot with no negative side effects.

Thoughts? Suggestions? Ideas?

Also looking for additional ingredients i could use to really spruce this up.....