It's not. We chose this baseline sort of by default based on the practices of some other major carriers. Your question is a good one, and we'll take it as feedback.
A few Mbps would be nice - fast enough to make the modern web mostly usable. 256 Kbps is almost the same as not working at all.
I would be a lot less worried about signing up for that plan if I could soft-cap myself at 10GB until I login to the app and push a button that says "yeah for real I'm going to use another 10GB of mobile data", so that if iOS goes bonkers and tries to download my entire 90GB iTunes library over cellular, it doesn't fuck me over for a month. I haven't exceeded 7GB/mo intentionally for years, but it's happened twice so far against my express wishes, and carriers are uniformly awful at that.