I believe Cirth and Tengwar are just less prioritized, unlike Klingon which was explicitly rejected (due to the non-usage of Klingon speakers at that time). Maybe it just needs a re-submission of the 1997 proposal by Michael Everson.
Klingon has a much more complicated history than this. The usage of Klingon is higher than many of the obscure things encoded in Unicode, so that's not the full reason. The real reason is most likely that Paramount considers the Klingon language its intellectual property and has attempted to enforce it[1]. Thus, I believe the official position of the Unicode consortium is that Klingon is written in Latin script but with a Klingon font (ie is basically a simple substitution cipher with a handful of ligatures), which satisfies everybody except for those who want Klingon included in Unicode.
ETA: The proposal[2] has been revived recently, and in the committee meeting[3], they're clear that the blocker is legal.
Ah, thank you for pointers to the revived 2020 proposal (my radar to the L2 register was partially off at that time). I'm not sure that Klingon the script was in broader use at the time of the initial proposal though---there are several constructed scripts that enjoy uses comparable to Klingon's and I don't think they have been well received. Or alternatively, they all actually have a potential to be encoded but only Klingon speakers are enthusiastic enough to pursue.
