Both have contracts expiring this year. Both are totally serviceable NHL players in their roles. They get thrown into trades, as both become redundant for the Flames. If the Flames acquire another centreman- then they don't need another top 6 centre like Monahan. For example, Eichel-Lindolm-Backlund is a very good top 3 center group, where does Monahan fit there?
Zadarov is not needed on the Flames, as Kylington has emerged as a legit #4. Zadarov was brought in to be a #4-5 to fill in while players like Valimaki and Kylington developed. Kylington is good to go now.
Neither of these players are "crappy" and would solidly fill roles on many other teams in the NHL.