From CSTART Wiki
 Why did you decide to use a hybrid rocket?
A hybrid design was decided upon mainly for fuel-related factors.
Solid fuelled rockets offer attractive simplicity, but in order to get a fuel powerful enough to reach orbit in the shape and size that we wanted, we would have to manufacture that fuel ourselves. This would be an expensive and dangerous operation, probably requiring government licensing. Furthermore, mistakes made while casting solid rocket grain can cause rockets to explode in flight.
Liquid rocket fuels and oxidisers, on the other hand, can simply be purchased commercially, rather than manufactured. However, the vast majority of liquid fuels and oxidisers tend to be cryogenic (requiring storage and handling at extremely low temperatures), poisonous, toxic or carcinogenic, or explosive upon contact with common substances like air, water and rocket scientists - and sometimes several of these things! Safe storage and handling of these materials would make our work slower, riskier and more expensive.
In contrast, hybrid rockets can use oxidisers like gaseous oxygen or nitrous oxide, which are safe, easy and cheap to purchase, store and handle. We may still have to manufacture our own solid fuel, but this is cheaper and less dangerous than manufacturing solid rocket fuel (since hybrid solid fuels are typically inert) and there is more margin for error (irregularities in the fuel grain are less likely to lead to explosions in hybrid engines).
A non-fuel related advantage of hybrid engines over solid engines is that they can be throttled, stopped or started.