The Thuggee escaped detection for so long for a number of reasons. As mentioned, they were highly organised, methodical, cooperative and used team work. They also ignored travellers on shorter journeys, no matter how rich they were. If someone was expected to arrive at a destination in a short amount of time the alarm would be raised quicker if they didn’t arrive.
By concentrating their efforts on people who were moving longer distances they got a huge head start before the alarm was raised. If a traveller was heading a few hundred miles and were a few days late, no one would think anything of it. By the time people came looking for the traveller the band of robbers would be long gone.