Though innerspring mattresses account for about half of the mattress market, there are fewer quality options available because they are more difficult to manufacture and ship. Because of these constraints, very few of the online-only companies end up with an innerspring option. That will probably change over time, but it is just much easier logistically to have a foam mattress if you are selling online.
Fortunately, the online-only companies that do choose innerspring tend to be more advanced in their mattress knowledge, and deliver a quality product (generally) at a really good value. You can really get a great product at a great price with these mattresses that should last a long time.