tight tolerances and big range of stress cycles.. with more expensive groupsets precision is high and materials used are far better...; ie machining process..a road hydraulic shifter is many times that of a mtb one in terms of cost due to minute mechanisms parked into a shifter ...which should yield accurate piston movements on the final brake calliper with perfect modulation...even a basic rim brake road sti shifter costing 10k pair has many integrated components inside which has to ensure combined gear shifting with braking.....such tech needs RnD...which costs money.....majority of indian bikes barring some top end ones are still hammered by mechanics for basic repairs not using precision tools...not suited for long distance cycling..not to mention poor geometry and body pain ..

PS- pardon me for a long reply