After discovering on Muppet Wiki that fresh episodes were added to HBO Max a couple of weeks ago, and I read on these forums offline that there seems to be a trend that newly-added episodes have a common factor (story arc, or the best of episodes featuring a certain character), In the initial batch, we've seen episodes in which Bert or Cookie Monster was a central character in street scenes. Bert was central in at least three episodes, and Cookie Monster was central in at least three episodes). This time, we've got episodes centering on Barkley, Tarah and/or Linda. However, I don't see anything with the 2003 episode with the Bert's birthday plot that involved those characters (Linda wasn't involved, as she retired in 2002).
Maybe they're saving the episodes with plots involving Big Bird as the central character - more along the "best of" episodes - for last.