The reason is, they set a standard for useless gimmicks which are far, far too expensive, designed to appeal to style over substance.
The OP1 is one of the most over-rated 'instruments' out there. It has a fancy, expensive OLED, a fancy, expensive casing, and useless gimmicks. The OLED never really shows you anything useful to the act of music-making. This is true of their Pocket Operators as well - its nearly all stylistic whimsy over functionality.
Save yourself the hassle and frustration of using a Teenage Engineering product and either buy the parts and make yourself an LMN3[0], or invest in devices that don't take the piss out of the user, such as the 1010Music Bluebox or Synthstrom Deluge.
The musical-instrument industry is rife with people who want to rip off the punters, who they know for a fact are easily afflicted with GAS (gear acquisition syndrome), resulting in customers across the globe who end up stashing their expensive, sexy-looking (but functionally retarded) toys in the drawer after a period of glib usage.
[0] - https://github.com/FundamentalFrequency
[1] - https://1010music.com/
[2] - https://synthstrom.com/