Probably because UPS and Fedex will charge you $8 to ship a pack of cartos via ground and take a week to deliver them. USPS will deliver that same box in 3-5 days for $2.50 or in 3 days for $5. Some vendors do not offer USPS. I will not do business with them unless my order is over $50 or so and there is no alternative.
Over the last 3 years, I have kept records about delivery times (from FL) and incidents of mis-deliveries and damage. USPS beats UPS with a record of 0 mis-deliveries in well over 5000 shipments and only 3 damaged items. OTOH, I had 2 damaged items and 3 mis-deliveries in the first (and last) 100+/- packages I shipped via UPS. That's about a 5% screwup record. Plus, they cost more and they're slower. It's pretty much a no-brainer to me. Oh, and I won't even mention the PITA it is to get a damage
insurance claim resolved by UPS. So I understand completely why a vendor wouldn't offer UPS. I sure won't.
Huh, I've had the exact opposite experience (though I use FedEx a lot more often than UPS). I've gotten packages from FedEx in 36 hours, from Hawaii, New York, all over the place, and I'm not just talking about PV related stuff. I primarily use FedEx for all my shipping stuff; Ebay, Amazon, etc. Granted, that is with two day delivery, when it's affordable, but from many retailers, especially big name retailers, it's not that much more expensive. I'd rather pay the extra $5 and have it sooner.
With USPS I can usually count on at least a 4 day wait, depending on where it ships from. I've waited, on occasion, up to a week. Though I am sometimes surprised; it does sometimes come sooner. I've never had a package lost or damaged from any shipping company. Granted, I probably don't go
through as many orders as you do.
I'm not saying UPSP is bad. I've used them, without problem. It's just in my case, I've noticed FedEx is faster. That may not be for everyone. I think it definitely depends on where you live, and where you're having your packaged shipped from. Sure, if you're ordering from the next state over, it'll probably be the same time no matter what service you ship with.