When you consider how bad BW were the Sequels not selling made sense. It’s not like they did anything to fix the starters. Heck, even the anime for BW got shit on for resetting Ash hard.
D/P had a lot of issues. I can see why Platinum did not sell well.
Same with R/S and Emerald even if they openly and obviously fixed their base games. Too little to late.
The starters all being kind of bleh hurt BW as well. Let's look at the previous generations:
1.) Charmander wins this one full-stop. A somewhat awkward middle stage doesn't hurt it at all. The Bulbasaur line is kind of nice too and in other generations would win out. The Squirtle line is hurt by Blastoise always feeling out of line with the others, and the Kanto Renewal Project mod gives Totartle which would have been good competition for Zard.
2.) The Cyndaquil line was also the clear winner here - there's a reason Whitney is the brick wall for everyone. The Totodile line is also pretty solid competition, but it's hurt in gen 3 until gen 6 by physical being its main stat and its appearance asks for it to have a part dark type. While I like the Chikorita line aesthetically nobody can deny that Johto in particular is
cruel to the line.
3.) This is the first generation where all the starters can have some argument for being best. The Treecko line was admittedly let down by Sceptile not really meshing with the looks of its prevos and an odd movepool that wants it to go mixed, but it gets a lot of help by staying relevant through the whole game (in Sapphire and Emerald). Mudkip's line is also fairly good and when it starts evolving it gets one of the best ingame typings in Water/Ground. It becomes especially good in Ruby thanks to all the fire types around. The Torchic line, despite not having a team it's good against, has a lot to work with. Its weak points in the gym challenge (besides Roxie) all hit late enough that you should have other team members to cover up, and it started the Fire/Fighting meme by being really good with it.
4.) This generation is won so damn hard by the Chimchar line that it's not even funny. Infernape is somehow a
better Blaziken and this is a generation that in the vanilla games has a massive lack of fire types. The Turtwig line is a little unfortunate since the Ground type makes it neutral to water types which does rear its ugly head and it overall comes off as a worse Mudkip line, but natural Earthquake is a very nice thing. The Piplup line looks really good and it has a smooth transition into Empoleon (with one of the best typings around) but it has one of the most unfortunate learnsets of all time and it demands TMs and Surf to come into its own.
Generation 5? Aesthically Snivy is a nice line but it is let down by the final evolution being a fast defensive mon with a horrible movepool (there's a reason Blaze Black gives it Contrary
and the secondary dragon typing, it needs that much help). Oshawott is hurt badly aesthetically by a really bad looking first stage and its final evolution drops the ball by going to all fours. Its stats and movepool are all kind of average and it would have benefitted from a lot more speed. Tepig is also where the Fire/Fighting typing turned into a hard meme and its stats are just awkward. A lot of HP, higher attacking stats... and poor defenses and speed. This generation has no really good starter that stands out and it hurts it a lot, while every other one had one people could rally behind.