A new phone, a new round of cases. Because with the cost of an iPhone, it’s almost foolish to not protect it. Case manufacturers have a tough job, needing to keep up with the latest models, changing every couple of years. But Apple has made things a lot easier for them recently. Mast cases for the iPhone 6 also fit the 6s, and the new iPhone SE can use cases for the iPhone 5 and 5s. This means that case makers don’t need to come up with a new line of products, and can keep selling cases that – in some cases (see what I did there?) – they’ve been selling for several years.
This is actually a good thing. If you own an iPhone, and upgrade it, it’s practical to be able to use the same case, especially if you’ve found one that you really like. You can’t always find the exact same type of case for a different model. It also makes things easier on manufacturers, who don’t have to redesign cases, and easier on the environment, because cases aren’t chucked after every upgrade.
There’s really no reason to change form factors with every new phone model. Apple has made the right choice with the new iPhone SE, using an existing form factor. Cases are reusable, and it probably saves Apple a lot of money too, not needing to re-tool their assembly lines (at least for the outside of the phone; the innards are a bit different).