I am going to ask you one question about your favorite episodes from each show:

TNG

Conspicuous lack of Romulan episodes. Didn't care for them?

DS9

"Duet"! How can you not love "Duet"??

VOY

Love of Tuvix aside, did Janeway do the right thing for the greater good, or is she a contemptible murderer?

ENT

No mention of the last episode, which is super polarizing. Good way to cap the franchise, or pandering snooze-fest?

Also, to answer your question:

If Andorians are one of the founders of the Federation, why are they not represented in other Star Treks?

The Tellarites were also founders and they are represented even less in TNG/DS9/VOY. They have some representation in TOS, but when TNG started they made a conscious choice to not focus on the Vulcans, Andorians, Tellarites, Klingons, or Romulans, who were the most prominent non-human races in TOS. The Vulcans, Klingons and Romulans eventually worked their way back into the lore, the Klingons particularly so thanks to Worf, but the Andorians and the Tellarites were still left out. So when Enterprise came around, they made a conscious choice to tell Tellarite and Andorian stories as a TOS callback.