iPhone is a fading product. They have no inovation and every release is the same, just a little bit of trim added or removed. Why not make cases for oh Note 4 or Note 3 or other phones like HTC One? Make cases for phones that are going somewhere.

Also just so you know these cases will not protect a phone from being dropped the simply offer scratch and scuff resistance. If you put a phone in an ABS or PLA case and drop it, nearly all the energy of impact will be transferred to the phone. If you want to make a case that will sell then redesign and include a silicone or TPU sleeve the case will fit over. Take a look at Otterbox. They guarantee your phone from any normal damage from drops and such as long as it is in their case. They will replace it if damaged. Now that is how you sell a product, be willing to fully stand behind it.

The reason you won't see many STL files for phones like the HTC, Samsung, LG, etc, is that they have curved backs with many compound curves, which are very hard to model.

So unless you have the actual drawings, showing all the dimensions, curves and radii, you're going to have a really tough time making a case that fits properly, so everyone is making cases for the iphone, which is a simple slab with only the corners rounded. 

Most of the cases you see for sale coming from China, are not actually designed on CAD, rather they make a silicone mold and use it as the reference to build the rest of the case around.  Some use scanners to scan the phone and then build on that.  They don's really spend time to measure and model the actual phone mockup from scratch.

Some companies do actually spend the time to design their cases.  Companies like Otterbox, Incipio, Speck or Element take the time to come up with unique elegant designs, and they do charge a premium for them, but the bulk (80%) of all the cases a just basic shovelware.

Oh, one last thing...  Your comment about the iphone fading....  I got 3 words for you:  you're in denial

Does this look like the market share of a product that's fading?

Don't get me wrong, I'm not an iPhone fanboy, I personally use a LG G2 phone, but there's not denying the facts.  BTW, their market share was "only" 40% before the introduction of the iPhone 6, and it hit 50% afterward.  Fading, eh...

Also just so you know these cases will not protect a phone from being dropped the simply offer scratch and scuff resistance. If you put a phone in an ABS or PLA case and drop it, nearly all the energy of impact will be transferred to the phone. If you want to make a case that will sell then redesign and include a silicone or TPU sleeve the case will fit over. Take a look at Otterbox. They guarantee your phone from any normal damage from drops and such as long as it is in their case. They will replace it if damaged. Now that is how you sell a product, be willing to fully stand behind it.

You're marketing an easily made product... to makers. Doesn't that make no sense to you?

Not trying to be a dick but I wouldn't expect a high sales volume.