He deserved birthday celebrations as much as anyone else in the group.

One thing that would happen in this, Leonard invites the people he wants; Apart from Stuart and the gang, he also invites: His father, Sheldon's mother, Dave, Bert, Kripke, Siebert, the fans of Serial Ape-Ist who heard about how Leonard got Penny, Wil Wheaton and Penny's parents and siblings.

When Sheldon finds out that Leonard refuses to invite his mother Beverly (and with good reason since she never let him celebrate his birthday as a child), Sheldon goes ahead and does so behind Leonard's back because Sheldon has no consideration for others' feelings and because Beverly benefits him. This angers Leonard so much and he decides he is done enabling Sheldon and doing him favours, giving him advice or helping him.

At the party during speeches and toasts to Leonard, Beverly just ends up putting Leonard down with Sheldon and this angers Penny that she pulls Sheldon and Beverly away in private and yells angrily at them, (Sheldon for inviting Beverly when Leonard said no and because Sheldon only wanted her there and Beverly for trying to ruin Leonard's birthday when she never let him celebrate it as a child because it was her achievement and also how they team up to gang on Leonard whenever Beverly visits).

Beverly is then expelled from the party and Sheldon is reprimanded by Amy and tells him if he ever does anything like this again, he would be in serious trouble and their relationship would have consequences.