JammedStab
It's a generic limitation - 5 minute takeoff. 5 minutes is the baseline cert basis for the engine (limits analysis and the such). Since a derate can be anything from a few percent to 40 percent (depending on the engine/aircraft), it's simpler to just maintain the same limitation without attempting to determine if the derate drops it below the max climb/con rating.
As C100driver notes, most aircraft/engine combinations have "10 minute takeoff" as an extra cost option. However, at least according to Pratt & Whitney, you're only allowed to use 10 minute takeoff if an engine has already failed (it has to do with the engine life limit analysis - basically it's assumed that the actual use of 10 minute takeoff will be really rare).