Well, back in the day many of the people making buying decisions were tech enthusiasts who like the idea of upgradeability, etc. Computers were quite expensive, and people didn't want to waste money on a box which can only do one thing.

Besides that, "app store" was just not feasible with tech of the day.

When vast majority of customers do not care, you can ship a locked down device.

You can buy a hackable phone, but it's a niche