That's because he's not very exciting as a fighting game character.
When we look at 3rd party, there are SO many options that are much more fitting.
Plus, his connections to Nintendo aren't that big, I mean, I dunno my PacMan so someone can correct me on this, but did he ever have a major Nintendo release that has been hailed as awesome?
I mean, Twin Snakes is a remake of the original Metal Gear Solid game, the one that kinda reinvented the series and set the standard not only for the Stealth Genre, but for Story-telling in vidya. Furthermore, Twin Snakes is a goddamn good remake of MGS1, so Snake's inclusion is rather fitting. Also, I'm pretty sure ports of the first two Metal Gear games are on Nintendo consoles as well.
The best I can think of for PacMan is that his design was an inspiration for Mario's design (though I never got evidence on this rumor).
I mean, just looking at Namco alone, there are tons of other choices I'd much rather see get in before Pac-Man, from a gameplay standpoint of course, and the deal is, a lot of people neglect that because "hurr durr, boring is subjective." Yes, as a stand-alone it is, but in contrast it's not. I'm sure we can all agree that Motorcross is a much more exciting activity than fishing, even if you prefer the latter over the former. And the deal is Sakurai is giving a huge look at what characters can canonically do before adding anyone in this time around.
MegaMan, while tough to work with, had TONS to work with, so it's obvious he'd get in. I dunno, maybe Sakurai can get a stroke of inspiration with PacMan that neither me nor Namco were able to get, but I doubt he'd ever add him simply due to his importance to the industry. Especially when he's basically washed up. Smash isn't some trophy wall for characters, it's a fighting game first and foremost, everything else comes second imo.