Given that the chemistry of the actors was a huge part of the show's success, it stands to reason that you would drop anyone who everyone didn't gel with.
I just wrote another comment about this. Supposedly Jason didn't feel it with Heidi (Susan) and so the relationship was discontinued at some point. I'd say this specifically might be why the relationship worked so well on screen.
Sometimes what a project like a sitcom or a business needs is rather someone or several members who don't fit in. Because otherwise you start to suffer from creative and intellectual incest.
I read it on reddit that Jason didn't enjoy working with Heidi as in his opinion acting chemistry was missing. If I think about it that might actually be true but was probably contributing to why that relationship was so enjoyable to watch.
What a prick. The more I learn about the core Seinfeld group the less I like them. Googling Heidi Swedberg and looking at some pics of her it seems obvious why they didn't get along with her. She's a genuinely likeable and good-hearted person. At least that's my impression.
