>>/6660/
 >>/6661/
 >>/6662/
 >>/6663/
 >>/6664/
 >>/6665/
 >>/6666/
 >>/6667/
 >>/6668/
 >>/6669/
I think this explains things very well, in particular  >>/6667/ Though it could use a revision or two. 

 
-The fanbase had shrunk since 2016 and the hope for the movie trying to reunite the old fans was not a successful attempt
- The lack of promotion coming from Hasbro, relying too much on the word of mouth
- The short amount of time for the new characters. Only Tempest managed to domain the popularity polls and thus, ended up being the fan favorite of the bunch.
-This species was fresh but still old. People were used to seeing seaponies, just not in this form. So technically, you are seeing the same old same but with a new perspective. So they weren´t all that exciting because the fans were already familiar with the seaponies despite not seeing them before in the show.  

This is really good at sumerising what is a pretty complex situation where you providved a lot of background context. Almost feels like the Seaponies had just the right things going on to "fail". Though that is too strong of a word. 

More like decaffeinated. They are not failures but they is a lack of enthusiasm for all the reasons you stated. Mediocre doesn't fit for they still are well designed and that is reflected in the fact that they get an above reasonable amount of fan art but little in the way of focus from the fanbase.