4 reasons:
1/ Flap on thin airfoil makes higher lift, but in negative AoA so it will work standard way (flaps to elevator 1:1) only in case that elevator is very small (so that it has so low authority that negative pitching moment will overdo elevatotor lift and so model will fly at negative AoA) ... such model cannot be stable as we expect on out c/l models (because of low stab authority). In normal standard configutation of large tail making high static stability when model gets positive AoA in maneuver- say 5 degrees optimal for flat airfoil, where lift coefficient goes to 0.3, the same wing flapped with say 10 or 15% flap will fall down stalled at lift coefficient aproximately 0.1 - those numbers look very low, but remember it flies at sub critical RE numbers somewhere at 50 000. It works more like insect wings, than normal. And you can see that all insects have flat straight wings :- ))
2/ Outdoor models need flaps because they are too heavy for small wings, reason is, we need small wings for turbulent air to resist in wind, but able to make tight corners - that is not necessary indoor, you can use large wing without problems, so flaps are not necessay.
3/ Indoor has micro line tension (centrifugal force overhead is even smaller than gravity), flaps have large hinge moment (aproximately 5 to 10 times more than elevator) slow flight simply does not keep tight lines for flaps.
4/ Amount of tip whirpools depends on aspect ratio and lift coefficient. We fly in calm wind without wind which can blow them out, so making small wing with flaps making the same lift at higher lift coefficiet as large wing at lower lift coefficient makes thing worse. No problem outside in little wind, but nightmare in calm air indoor.