Something to remember;
DLC wasn't a thing till 2017, specifically by Nintendo, not Sakurai. That means an AT that was a character considered for base game couldn't have been "saved for DLC", since there was no way to know it was happening. He did not plan for DLC at the start.
Bomberman not being in just means he wasn't high enough priority over Castlevania. That's pretty much it. With Snake returning too, that's a third franchise that needs a lot of content. Sakurai also felt he needed two Belmonts to properly represent the franchise. Bomberman was very likely considered for playable, but asthat wasn't an option, he was hit with the AT status, something that happens here and there. I remember Sakurai noting that AT's can be for characters who couldn't make it into playable status.
Moveset could've played an issue too. Whether he couldn't see him dancing in his mind, or just that balancing it was very difficult. Though I honestly am of the opinion that Castlevania out-prioritized it(along with Metal Gear) and since DLC was not an option at the time, well... AT it is.
His Spirit was also treated as an important one in World of Light, so clearly he thought Bomberman was something special outside of being playable too.
Sadly he has yet to speak on a lot of characters who were probably considered. He hasn't spoken on Waluigi since literally Smash 4's base game, and that was more of a fun joke than actually speaking of it. He doesn't talk about that many characters he considers as is, and he probably can't anymore due to DLC being chosen by Nintendo. Any character he says can be taken the wrong way now, so he has to keep his lips sealed for business reasons. Once Ultimate is over, he's got more freedom. That, and some characters not working out could be due to companies clashing with each other, so he has to bite his lip on stuff to prevent any bad blood. As a business man, his words have clout, but moreso, he can't suddenly create chaos among companies just cause he wants to be honest about why someone didn't work out. Look at how little he spoke on Geno's problems during Brawl. Just "it didn't work out". If he said anything clear, that could be problematic. If SE, say, charged too much, that could make people not want to buy their DLC. Not something I believe would happen, but that's the kind of risk he takes when opening his mouth.
That sucks the Premium costume he got had no music, yeah.