Was ever explained why Pikachu lost VOLTEKKAAAAAAAA Volt Tackle for Electro Ball?
Well, other than the anime studio's collective bias for projectile attacks...
It was weird how it happened in the episode. In episode
BW020 "Dancing with the Ducklett Trio", Ash & the Unova crew encounter a trio of trouble making Ducklett who steals some of their stuff. Going after them, Ash tells Pikachu to use a Thunderbolt. However one of the Ducklett had flown over them with an umbrella and drops it on Pikachu, it closing and causing Pikachu to shock itself. At the climax of the episode, Sandile (who had its glasses stolen) is being attacked by the Ducklett Trio as Pikachu struggles with its electrical attack, Ash thinking the umbrella did something. Just then Pikachu uses Electro Ball, sending the Ducklett Trio blasting off.
And that's how Pikachu lost Volt Tackle and got Electro Ball, apparently Thunderbolt + trapping Pikachu to shock itself = Electro Ball replacing Volt Tackle. Like at least make it so Pikachu was using Volt Tackle when it got trapped, because if anything that more sounds like Thunderbolt should have been replaced. But they can't do that because that's pretty much Ash's Pikachu's signature move (even the Ash Cap Pikachu has it using Thunderbolt to use 10,000,000 Volt Thunderbolt), Quick Attack and Iron Tail are useful utility, so guess its Volt Tackle that gets the boot.
If you want a non-anime reason, I guess they just wanted to give Pikachu a new move from the current generation at the time. I would also maybe suggest Electro Ball was easier to animate than Volt Tackle but I don't know how true that is.
That all said, the Pikachu species does have its own Z-Crystal that requires Volt Tackle and they do have it so in the game you can teach it via tutor. I can maybe see Ash being given the Pikanium Z, told he needs Pikachu to know Volt Tackle, and they go through a training montage to re-teach Pikachu Volt Tackle (probably replacing Electro Ball). Then again, if they really don't want Pikachu having Volt Tackle, they could just have Pikachu use Thunderbolt/Electro Ball. Maybe I can also see them having a rival trainer with a Pikachu that uses Catastropika, Ash tries to figure out a way they can match that power (maybe having gotten a Pikanium Z but realizing they can't use it and don't know how/unable to reteach Volt Tackle), and they somehow get/create the Pikashunium Z (possibly from the Pikanium Z). Would also be a way they could introduce Hau, have him be the trainer with Pikachu who later evolves into an Alolan Raichu that uses Stoked Sparksurfer.