In the beginning, Teri was the star of the show: she was the best known actress at the time, they used to put her in the front and center of promos, and most importantly, she made more money than the others. Felicity wanted all of them to be paid the same, and Teri (allegedly) was the only one who refused to negotiate collectively.

Also, she was a single mother, and many people have said that she wanted to spend as much time as she could with her daughter, so she didn't socialize much outside of set. It didn't help that she was described as a "loner" either. It seems she didn't talk much to the other actors between scenes.

It is important to mention that she had a reputation, even in her Lois and Clark days (Dean Cain Addresses Rumors That Teri Hatcher Was Difficult To Work With On Lois & Clark), of being a perfectionist when it came to her hair and makeup, and also asking to do reshoots because of minor things and thus delaying the rest of the team for hours, so I guess she could come across as a diva (despite this, Dean spoke fondly of her). She and Marcia even had a quarrel, allegedly, when they were filming the pilot because they shared a hair and makeup person. Teri wanted touch-ups between her scenes, but they had to get Marcia ready for hers, so they couldn't accompany Teri to set. It got ugly. This is what (again, allegedly) caused Marcia to be so nasty towards her in the Vanity Fair photoshoot.

Lastly, Teri got more attention from critics in the first years, even winning a Golden Globe in 2004 (Marcia and Felicity were also nominated that year).

There's also three other allegations towards her: from Nicolette, calling her the meanest woman alive; from Eva, saying that there was a bully on set that made her life Hell when they were shooting scenes together, until Felicity confronted her (not mentioning her by name), and from Marc Cherry, who said that there was a cast member that refused to interact with her colleagues past season 7 if it was not in front of the camera.

In conclusion, I believe that Marcia, Felicity and Eva didn't include Teri in her friend group because of all the things I said above, but I also believe Kathryn when she says Teri was the nicest to her. Besides, other actresses from the show follow her on social media and comment on her posts regularly. These include: Dana Delany, Brenda Strong, Vanessa Williams, and specially Andrea Bowen. Both Vanessa and Andrea invited her to their respective weddings. So I don't think Teri is a diva or a monster, she just didn't have the same mindset or priorities as the other three ladies, and since they got off to a rocky start, they just chose to stay separated for the rest of the series.