Helical gears for the spur and pinion would require the use of thrust races in the system, adding to cost, weight and drag. When a straight-cut gear is in operation, the resultant force drives the gears apart against the bearings already in use in the motor and drive shaft.
When a helical gear is under load, the resultant force drives the gears apart in a side load/motion that would push the driveshaft against the drive cups. Some sort of thrust race would be needed to prevent it jamming into one end. Same for the motor, the helical will push/pull the armature/rotor to one end of the motor putting end loads on a bearing designed for radial loads.
A double helical spur/pinion would contain those forces and prevent side loads, but that's a complex thing to design and mould accurately.
If helical gears were used in the gearboxes (as bevel gears) then because the tooth contact is greater, the gears could be smaller. Smaller gears would reduce the drag. It's all about tooth loading. The more teeth in contact during power transfer, the smaller the contact area of each tooth needs to be. Helical bevel gears are used in applications where size is important (as they can be smaller than straight-cut) or where noise is an issue (because they are quieter).
Hypoid bevels are used where height is an issue, as the pinion can run a centreline below the centreline of the crownwheel. Hypoid bevels are usually used in automotive applications.
All in all, the straight-cut design is more than adequate to deal with RC loads, and we are not usually worried by noise or driveline height. That helical gears can be more efficient than their straight-cut equivalents is well proven, but the cost and complication for RC just isn't worth the effort. HTH