...Like did even a single person ask for the plant lol...
Smash isn't necessarily supposed to be trying to fulfill every wish list ever- That is impossible (Also is varies region from region and person to person so they'd never finish the game.) Remember that Smash has added characters from obscure series to add variety, expose people to new games, or let the development team stretch their creativity. Then they become more popular, end up being referenced in more games, and they become a Nintendo/Smash staple.
Besides, some of the more obscure characters in Smash are also pretty deserving on their own. Duck Hunt appears twice in the top 3 best selling NES games. The NES is iconic and so is the game's visuals themselves. Wii Fit and the Wii series of games are one of the best selling game series EVER. Most series that are even close to the sales of the Wii series are 20-30 years old. It was honestly a pop culture phenomenon similar to Pac-Mania, Pokemon-Mania, the Minecraft craze, etc.
As a design, piranha plant has an incredibly iconic look that many non gamers would easily recognize. Probably much more so than Link, Mega Man, Mewtwo, etc.
Sakurai said they were looking for a character to add who wasn't the typical protagonist/antagonist/deuteragonist character role that we have gotten time and time again. So even if they didn't settle on piranha plant as this character, it still probably wouldn't have been the character you were wanting... unless you have a thing for side and supporting characters.
...Bomberman, who was eliminated from being playable as an assist trophy...
That isn't how that works. They chose the roster when the project plan started. They didn't say "Hey, let's make this character an assist to piss of their fans." They actually are more like "Hey, we don't have enough time and resources set aside to make this character, but we really think they have fans/are important to the industry/Nintendo- maybe we can make them an assist because it takes less time and resources to do." It still takes time and effort, but not as much. They got added as an assist because of their fans and importance. Smash Bros will never have a roster containing every character somebody in the world wants, but they try and give nods to the more popular and important characters in many different ways- Assists, Items, Stage elements, etc.
period.
