always surpised how GBA flash cards STILL cost more then DS flashcards... must be tougher to manufacture and a specialized market
Actually it's DS flashcards that are much cheaper than average
And it's not (mainly) supply and demand either, it's:
1- hard performance requirements (DS loads everything in memory first over an encrypted and relatively slow connection, GBA software runs directly from the card - and unlike a CD-based console, most if not all software was never designed to work from slower-than-official roms or dirty connectors)
When the GBA was current generation, memory (whether SRAM or NOR) fast enough and in sufficiently large sizes to fit most GBA games were uncompetitive luxury products not differently than a half-TB SD card is today (and even without speed patches, still required save patching, as they do today, because not all 3 types of save were implemented)
2- Research costs: my original R4 was about 25 € in mid 2008 (a time in which apparently every phone/videogame accessory seller in Italian electronics fairs sold them, you had to manually build YSMenu for updates - they mainly sold on former reputation, and R4SDHC clones were new), two years ago you could buy them for 5 € freeshipped from China and enjoy one of the best DS flashcard kernels ever...
Not much differently, the newest EZflashes and Everdrives are expensive, especially if bought new, but older GBA flashcards (which do most of the final result equally as good, just slower and less conveniently) are cheap... if you can find them!
"Modern" DS flashcards don't really count, the hardware hasn't changed at all since 2014 (unlike what r4isdhc.com may want you to believe) and probably didn't change significantly from the start of the decade either
3- The post-console age: in the mid 2000s, combining all your original games (and some more), plus NES games, "toys" (Pocket Physics and World of Sand anyone?), multimedia, and even some limited but serious productivity software (calendar, email, ...) for a medium-low 3-digit price was a huge deal;
if on the other hand you just wanted piracy... still a console and flashcard it was, as DS emulation sucked back then (even today it's still not that accurate...), and while GBA was decently playable on a contemporary laptop they were nowhere as cheap and portable, of course!
Nowadays, on the "what GBA model is your favorite" polls, you always have people only-these-days-not-completely-trolling with answers like "PSP", "Vita" and "3DS", and this is on forums where most people value good ergonomics: I wouldn't doubt that a non-negligible amount of people play Pokemon fine on a touchscreen...
But if you want the original experience (and/or that weird accessory), what are your options? Yup, console and flashcard!
(OK, this point was "supply and demand covered with an excuse"
)