The services that they have

AFAIK, Cricket and iPhones are available.

We are all other users here and would not know why Apple does something or not - you would need to ask Apple.

Having said that, I decided to google Cricket phones and came up with this:

https://www.cricketwireless.com/cell-phones/smartphones

Seems Apple iPhones are being offered.

Cricket is an MVNO that uses the AT&T network. iPhone’s work just fine on it.

Apple does not partner with every single carrier to offer pre-configured phones for each. But you can buy an unlocked iPhone from Apple and use it with Cricket, or buy one from Cricket directly. If you include regional carriers and MVNOs, there are over 60 cellular service providers in the USA. Apple has chosen to partner with the four that own nation-wide networks (all others are either regional networks only, or MVNOs that actually use one of the big four’s physical network).