Tumbling (Step by Step)

Obviously, out of universe the reason is that the show is establishing its rogues gallery, but I think you can make sense of it in universe too.

Basically, Zurg is still recovering from the events of TAB. That seems to have been his biggest scheme to date, he probably threw a lot of resources into it, he attacked what was presumably an important Galactic Alliance planet to steal the Uni-mind which almost forced a full-scale confrontation with Star Command (something I get the feeling both sides are wary of since they're pretty evenly matched), and he might still face retaliation from them for that and for everything else he did, plus his number one agent has been arrested, his tower is a wreck, and Star Command even occupied Planet Z briefly after he escaped in the Dreadnought (presumably he drove them off again, but who knows what kind of damage they did while they were there).

So while he's building his power base back up, he messes around with smaller-scale schemes that are very low risk for him because he's not directly involved, and that also don't require tons of resources. NOS-4-A2 was probably him testing the waters once his forces had recovered with something a bit more intense while still not being entirely hands-on in case it went south, and then Planet Destroyer is when he actually gets 100% back in the swing of things. From then on, most of his schemes are much larger in scale and he's usually actively involved (for example, Lost in Time, Rookie of the Year, Stress Test, Super Nova, The Lightyear Factor, Star Smasher, etc. which all have him striking directly at the Galactic Alliance in a very visible way).

Part of how Stephen Schwartz became acquainted with the novel of Wicked and came to see its potential as a musical was that in 1996, he was working on The Hunchback of Notre Dame with Demi Moore voicing Esmeralda, and at the time she was in talks to star as Elphaba in a film version of Gregory Maguire's then-new novel. That movie was never made, but the inspiration stayed with Schwartz.

Schwartz has also said that a main inspiration for the choice to focus on Elphaba and Glinda's friendship was The Prince of Egypt, which was released just around the same time that Schwartz and Winnie Holzman started work on Wicked in earnest. The way The Prince of Egypt gives the epic story of the Exodus a human heart by focusing on Moses's relationship with his adoptive brother Rameses, showing the tragedy of two people who love each other but end up on opposite sides of a conflict where the fate of a nation is at stake, was what Schwartz and Holzman realized could also make Wicked work onstage.

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