Inventory Management - its not that hard.
Especially in 2019, after the advent of spreadsheets, inventory tracking tools, etc. and they can track what each store has moved for the last 5-7 sets, each region has moved, factor in Christmas sales, etc..
Speculation for the decision? This just smells of bad management, or management turnover. "Never attribute to malice that which can be adequately explained by stupidity."
Unfortunately, and ultimately, we the players pay for this. Vastly rising/falling card prices in new sets (ahem Jirachi) that crash a few months later (ahem Pink Ninetales GX) cost the players who flock to new competitive decks and cards. And if you have kids (I'm still paying for my kid's 4x Rayquaza who spiked at $25 in September)...