They didn't want to call it the Nu variant. Xi would have been appropriate if China didn't control the WHO.

IT'S A CONSPIRACY BY DR FAUCI AND THE EVIL SOROSIAN STANIC REPTILOIDS FROM PLANET DEMONICRATIANDIA TO SCREW TRUMP OUT OF HIS RETURN TO THE WH YET AGAIN NEXT WEEK FOR THE NTH TIME!!!!

That's a good question. I just assumed they were. It would make sense that they followed some order rather than randomly picking and choosing Greek alphabet characters.

They do name them in order, just not by discovery. When it becomes a designated VOC, it gets a letter. If it doesn't become VOC, then (usually) no letter. So even though they discovered it, say, March, they may not designate a letter until July when they decide it's a VOC.
Basically, they only tell you about the ones they want you to worry about. Here's all the variants as of last time I checked. You can find info about them much easier with these.

B.1.1.207
B.1.1.317
B.1.1.318
B.1.1.519
B.1.1.523
B.1.1.529
B.1.1.7 = Alpha
B.1.214.2
B.1.351 = Beta
B.1.427 = Epsilon
B.1.429 = Epsilon
B.1.466.2
B.1.525 = Eta
B.1.526 = Iota
B.1.616
B.1.617.1 - Kappa
B.1.617.2 - Delta
B.1.618
B.1.619
B.1.620
B.1.621 = Mu
B.1.621.1 = Mu
B.1.630
C.1.2
C.37 = Lambda
C.36.3
CAL.20C
P.1 = Gamma
P.2 = Zeta
P.3 = Theta
R.1

AWESOME!!!!!!!! Thanks for that information. Up vote for you!!