Are DPIs better than MDIs?

Your question:  Why aren't any rescue medicine available as a DPI

My humble answer:  A couple reasons: 

1. The powder can induce an asthma attack or make asthma worse
2. A flow of 60 LPM must be generated to disperse the medicine into a fine spray 
3. Patients with severe lung disease may have trouble generating enough flow
4. Patients with asthma attack may not be able to generate enough flow
5. HFA inhalers get deeper into the lungs than DPI
6. The devices used to deliver a powder are expensive 

Because of the high flow demand required to disperse the powder, patients with neuromuscular disorders, the young and the elderly may not be able to use DPIs.

However, DPIs are easier to use than MDIs and therefore solve the coordination issues prevalent with MDIs.  No propellant is needed, so environmentalists like them better.

An Albuterol MDI was introduced to the market a few years back and was taken off the market a while later basically for the reasons above.

DPIs work well only for people with normal breathing, and that's why many preventative medicines are available as DPIs.  However, new evidence that HFA propelleant gets deeper into the lungs than DPIs, and therefore many physicians are getting away from prescribing DPIs.  As you will note, Advair is now available as an MDI just for this reason. 

Your question:  Do DPIs work better than MDIs

My humble answer:  No.  Every study I've ever seen show that MDIs, DPIs and nebulzers, when used appropriately and as recommended, all provide relatively equal distribution into the lungs.