One example: power creep. Making newer stuff more powerful than older stuff. Why do that? Because it sells books. It's bad design because it makes the game unbalanced, players stop using the older stuff, and pushes players to newer stuff...but it sells books. Good design would be to meticulously balance the game, bad design would be to progressively unbalance the game to sell new books with busted PC options. Have a look at the PC options in Xanathar's compared to the base options in the PHB. Then compare the options in Tasha's to Xanathar's. Neon-sign glaringly obvious power creep. It's a common tactic and has been going on in the RPG industry for...ever really. The first supplement for OD&D introduced paladins, thieves, and STR giving fighters to-hit and damage bonuses. Splat churn. Power creep. Etc. It's one example. There's more.
There's another great example: D&D 4E. It was a wonderfully balanced game, meticulously designed (after a few early hiccups), but it didn't sell well enough to meet expectations. It sold better than every other RPG on the market (with the possible exception of Pathfinder for a few months...maybe), but it was scrapped because it didn't sell enough for WotC. Great design + bad sales = scrapped game. Sales matter more than design. WotC is in the sales business. Good game design is secondary.