Pre-paid plans, when they came out, were marketed primarily to individuals who couldn't pass a credit check to go on the flexible, post-paid plans. This is why they were second class, pre-paid was like having to go to a check cashing place as opposed to having a checking account.
Suddenly, in the mid-aughts, that changed. Expensive, "hot" phones came out, like the Motorola RAZR or later the iPhone, and the marketing for post-paid plans were that you would get a free phone if you signed a contract. All of a sudden, the flexible option was now primarily a financing option for getting an expensive phone.
Meanwhile, post-paid services like Virgin Mobile became a bit more upscale, offering reasonable mid-range phones and decent pre-paid options. Still, a large number of people kept using post-paid plans to do monthly financing on expensive phones. "Unlimited" plans started to make pre-paid convenient.
The contracts, financing options and "family plans" have all been so profitable for post-paid carriers that they've basically done everything possible to give the impression of being the "first class" option - visual voicemail, 5G access, less throttling, etc. These have trickled down to some pre-paid carriers - Mint, Cricket and Visible generally offer all of the frills of the post-paid ones these days - but there is still the perception of them being "for people who can't pass a credit check".
Edit: often, if a carrier offered both pre-pay and post-pay, the post-pay options were designed as second-class citizens on the carrier's network (ie, throttled, or with different payment plans) for the reasons above, and as such operate substantially differently than post-paid both on the network and in terms of support.
I can easily pass any credit check, but I use a nice, high-end used phone and a yearly Mint plan and pay a third of what virtually everyone else I know pays while missing out on absolutely nothing. Suckers.
Counterexamples: Google Fi and Mint Mobile, both of which have good websites/apps, service, and product features. Verizon in particular sucks even for subscribers so I wouldn't expect their prepaid service to shine.