There seem to be two reasons for Linda not being included in this dinner date that the boys organise with their father. The first is that ostensibly this is a male occasion to celebrate the hoped-for success of Biff's meeting with Bill Oliver and the start of his new business. We need to remember that we are presented with a male world where women were often excluded from their discussions about business and plans for the future.
However, also, I think we can say that Biff and Happy don't include Linda in this dinner date because they are essentially rather selfish characters and overlook her. She, of course, is all too willing to be overlooked and is happy for the boys to spend some time with their father in the hope that Biff and Willy can repair their relationship. However, throughout the play, Linda is a character who is taken for granted by those who should love her the most and should appreciate her and her sacrificial way of living her life for her husband. The fact that she is not included and left by herself for this family celebration reflects this.