I'm writing from memory but if I recall correctly, Shriver said in her diary that Fed Cup usually conflicted with her schedule and, besides, she normally played Wightman Cup so she felt that she had done her job. In 1985 she chose to play Filderstadt while Garrison, Jordan, Walsh and Burgin went to Nagoya and lost in the final to Czechoslovakia. Burgin had to replace an injured Garrison in the semis and the final and Pam had a poor choice of words when she said, maybe to Burgin, that the US was sending her B team again
In 1984 Fed Cup was played in Brazil and on clay so Shriver had no business being there. The team, which lost to Australia in the SF, was formed by Horvath, Jordan, Garrison and Smith.
In 1983 it was played in Zurich and on clay again. Jaeger, Candy Reynolds and Paula Smith were the team.
In 1981 Evert, Jaeger, Casals and Jordan won the cup and Navratilova, Evert and Leand defended it in 1982. Evert also played and won in 1980 with Austin, Jordan and Casals. Out of these three, only the 1982 Fed cup was played on hard courts.
In 1979 Pam was in the middle of her slump and Evert, Austin, Casals and King made it to the team which won. The year before that same team clinched the cup in Australia but I suppose Pam was not selected.
SO my guesses: Chris probably chose to skip Fed cup after winning it six years in a row. Austin played and won the tournament three years in a row and in 1981 she was already playing a selected schedule due to recurring injuries. Jaeger was a substitute in 1981 and 1983 and Martina played in 1982 so Pam had no singles spot there. Also, I guess Pam wasn't considered for doubles until Rosie's decline since the latter was the more experienced player and Pam was just starting her remarkable partnership with Navratilova. It's weird that she didn't play doubles in 1982 but maybe Leand was chosen as a possible singles replacement. Navratilova and Evert played most of the doubles anyway so I suppose that was the plan all along.
As for Martina. I think she wasn't eligible to play until 1980 or 1981 due to her nationality issues and I don't think she felt too tied to the competition at all since she only played here and there and because of certain reasons. So, in 1982 it may have been the first time she could represent the US team on American soil, in 1986 it was her return home and in 1989 it was Evert's farewell. From that moment on, Navratilova only played in order to get in the Olympic team in 2003 and 2004 and a random tie in 1995.