They're releasing about 1-2 books a year and Psionics has always been a fringe subsystem with little official support.
I respectfully disagree. I consider that lack of an official psionic system, whether in a dedicated book or as part of another book, in a major published release, a huge ball drop for 5E. I do not count UA. Lets look at the game's history:
1E (One year from first release to Psionics release)
1st Book Released: Monster Manual (1977)
Psionics Released: Player's Handbook (1978)
2E (Two years from first release to Psionics release)
1st Book Released: Player's Handbook (1989)
Psionics Released: The Complete Psionics Handbook (1991)
3E (One year from first release to Psionics release)
1st Book Released: Player's Handbook (2000)
Psionics Released: Psionics Handbook (2001)
4E (Two years from first release to Psionics release)
1st Book Released: Player's Handbook (2008)
Psionics Released: Player's Handbook 3 (2010)
5E (5+ years and no current Psionics release)
1st Book Released: Players Handbook (2014)
Psionics Released: ??? !!!
Don't forget Original edition!0E (Two years from first release to Psionics release)1st Book Released:
Volume 1 Men & Magic (1974)Psionics Released:
Supplement 3 Eldritch Wizardry (1976)
Let me make another observation.
Gygax wanted a Medieval game of fantasy. He is on record as stating that both the monk and psionics didn't fit his vision for his game. Those thing were originally included due to popular demand, and because Gygax at the time like including things that others wanted, even if he didn't want to use it himself. In the same statement about the monk and psionics, he indicated that he planned on removing them from the game in the next edition... however when the next edition came out, both things remained, and Gygax never (to my knowledge) mentioned exactly why he changed his mind. Regardless, he used neither in his personal games.
Despite that, psionics was and has remained popular throughout the game, and I for one am a fan of the concept of psionics, if not always of how the game executes said concept of psionics. In the numerous additions to psionics in 0e, it was often used as a method to grant otherwise non-magical people with the non-magical abilities they displayed in books and movies. IIRC, Conan himself was originally listed as having "unconscious use of psionics" to explain some of his superhuman feats of strength, endurance, and recovery.
Personally, I think it is a great concept that is deeply embedded in fantasy (indeed, the famous Appendix N contained several works of psi-fantasy), and should be included in every edition of D&D.