Why is Leslie still out of the picture?

I have no idea if Leslie has not being on the show in current episodes since her final appearance in the 3rd season finale prior to her now current appearance in Sheldon's birthday episode after 137 span in total? Who was thrown into the show to instantly replace her with someone else? Whatever it's true or not she had been for a long time or if Amy or another character was the one to replace her on the show? And where is she now? Who replaced her now as Sheldon's nemesis other than Wheaton and Kripke? I miss Leslie and she always called Sheldon "dumbass" and it isn't fair she isn't even there to call him that ever since that time.