Bowser has many of the really bad problems DK has as well.
Generally framedata for heavies is pretty bad overall. Even if they have some decently fast moves here and there, most are kinda laggy and slow on startup, landing and jumping (from the ground) takes more frames, etc.
Bowsers normal grabs have bad framedata if they miss (and even the startup for a normal grab). Having to deal with 2 frames faster grab during his fights just cries for the ones with faster grabs to get them more often.
Heavies are also big targets, and have terrible landinglag on aerials. This makes it so difficult for them to land. At least Bowser has a command grab for that. But if the opponent spaces his moves well it's still difficult to land.
DK and Bowser have a absolutely terrible time landing. Though DK even worse than Bowser, while Bowser might struggle more when he's on the ledge. They often have to retreat to the ledge just to be able to get back to the ground safely. And then they still have to get back up unharmed. And with such an obvious getup roll that DK has for example, it's a really though job too.
They can't use an aerial, because that will just be shieldgrabbed, they can't always use their sideB to catch shielding opponents offguard (sometimes it can happen, but for DK a small shield will not stop the opponent from harassing him some more in the air in non-shieldgrab ways). How in the world are they supposed to land? Oftentimes I'm hit as DK without my doublejump and not hit far enough to be able to reach the ledge while falling down. This means the only thing he can do is hope the opponent messes up, or just get hit (his aerials can be shieldgrabbed, his sideB only shieldbreaks if it's already somewhat weakened). He can literally not do anything else. In Brawl he could've retreated with an aerial upB onto a platform with no lag, but now he even has lag on a good landing.
And of course on top of that their aerial mobility is absolutely terrible. B-reverse is possible, but even that won't help most of the time.
They're also big and easy-to-hit targets because of that. While making them easier combofood.
All of this together and most of their heavyweight-advantages yield much worse disadvantages for them.
And while some of their smashes are really strong, they more often than not have to rely on their tilts or aerials or something to get KOs, because their smashes are made so slow (I guess with Bowser it's a little different, because of his super good invincibile usmash). While their other moves don't really create setups to "frametrap" their opponents into them.
DK now has a superlegit combo that works most of the time against non-floaties, while even being able to KO most of the cast at like 80 % (depends on stage, weight, rage, staleness, etc, of course), which is a huge buff for him and no other heavyweight had this before (talking about Smash4). Finally one heavy has something that heavies usually don't have (which is another reason why they're generally bad characters). He has his own hoo-hah.
In Brawl Dedede was a special heavyweight too.
He actually had pretty fast framedata for some things. Like grab and stuff, while having that superuseful Z-axis dodges which made him super difficult to hit, while being super scary if you miss your stuff or he shields it, because he can grab you most of the time, while additionally gaining so much reward from that grab. He also couldn't be juggled for free like DK, because of his many jumps and inhale could catch off guard as well (while having that precious grabarmor). In Brawl characters could also land easier through airdodges, but now in Smash4 even that option was taken away from them.
So, heavies are usually just made to be bad, because Sakurai wants them to be slower, laggier, bigger, without many combos and bad mobility. All these things together are pretty bad.
Brawl Dedede and now Smash4 DK at least have some of the things "normal" characters have. But they still weren't / aren't that great characters. Because they're inherently made worse.
When they finally realize that heavies need a few fast attacks and some form of mobility or at least a few of the better aspects of characters we might see some rise higher than before.