Kirby's D-throw was not nerfed in Melee because Smash-64 only had two throws (Ninja Drop and Suplex). And you say his D-throw and U-throw are "strong compared to his other two (throws)", but I don't see how you came to that conclusion. They will only be doing about 8-10% damage, and you cannot combo out of it if the opponent is anything but a poor gamer. Teching the D-throw is extremely easy and DI away from Kirby after the U-throw is a walk in the park.
Now, this forms the basis of my argument as to why the U-throw and D-throw are the two throws that I would like to see buffed of all the throws. According to game balance theory, if an attack is especially easy to avoid or protect oneself again, it should have a greater degree of damage on someone who fails to avoid or protect oneself in the given time frame. Neither of these throws have that. In fact, it's quite the opposite. It's quite possible, and often likely, that if you use a U-throw or a D-throw the opponent will have the opportunity to hit Kirby before Kirby can attack or even defend himself.
Something this ridiculously disadvantaged would need something fairly strong to make up for it. What's nice is that the U-throw can Kirbycide... but that requires you to be at the very absolute edge of certain stages against certain characters, and even when it works Kirby will Self-Destruct along with his opponent (Kirby SDs first, so he loses when both are on the last stock). We could say this makes everything fairly balanced since it's still an advantage of sorts, but as far as an actual throw, it still fails pretty hard since 99% of the time you're just going to be doing about ten damage to your opponent and that's all.
The D-throw has the same problems except you can't even get Kirbycides with it. One advantage is that the opponent might not tech it because Kirby is such an underused character they don't know the timing for it. I emphasized "might" with italics because the timing is fairly obvious and most smashers completely unfamiliar with Kirby will still be able to tech after the first D-throw, and with it's long start-up animation it's not like you're going to be taking them by surprise with it either.
The other use D-throw has - the one that Kirby players know to take advantage of the most - is using it to tech chase. This is sort of funny because Kirby is so slow he really can't tech chase; you just pick a direction and go there. But we all know this does not work on floaties because they can hit Kirby before he completes the animation of his D-throw - like Luigi can just N-air Kirby after a D-throw. This will of course either set him up for a combo or kill Kirby outright depending on the damage. Not a very favorable outcome for an attack that most characters use to setup their own combos or put their opponent into a strategic position.
Now, looking back at Kirby's F-throw and B-throw we see they have the same inherent flaws in game balance that his other throws have: easy to DI/tech to prevent any further combos. But these these throws are Kirbycideable throws since Kirby moves an obvious distance to the side when executing them, hence why they are so easy to break out of (balancing instant SDs with chances to escape). But these throws can be used to direct your opponent off the stage, something U-throw and D-throw cannot do since they are mostly vertical. This makes the F-throw and B-throw Kirby's most useful throw... well, the B-throw anyway, the F-throw takes so long most people are able to break out of it. But that gets me to another point.
When an opponent breaks out of the F-throw or B-throw, Kirby will often have enough time to get a hit in on the opponent before they can properly defend themselves. Maybe not a smash, but definitely a tilt, jab or b-air, and then follow up from there. This means you can actually end up doing more damage to the opponent than had they simply not broken out of the attack to begin with.
So, looking back at the advantages, drawbacks, and uses of each of the throws I've given, I'd like to ask to go into a little more detail as to why you don't think that the D-throw and U-throw shouldn't be buffed over the others?