While I think good points are being made about how they see Laura as too matronly, there's more to the story here.
Felicia and Lucy are played by women who have maintained their tight, athletic bodies, who can still wear revealing, fitted clothing, who can still have their sex appeal played up. It doesn't happen. Lucy and Felicia get far less airtime and onscreen romance than Laura did. Neither actor is on contract. Neither actor is allowed to mix it up with the leading male characters. So it's not that.
Tracy spent large periods of time on this show alone. Her sex appeal was rarely played up anymore. She was dressed in the epitome of stately, matronly wear. Jane Elliot never lost her contract.
There is more to this story.