Operationally, they found that with careful coordination of the boarding process a single rider line wasn't necessary. Much like with Soarin' (which had Single Rider when in test and removed it within weeks), a properly aware CM can coordinate party sizes to fill every seat on every cycle of the attraction with relative ease. As a result, the Single Rider line would end up moving as slow, or slower than the normal standby queue. Single Rider typically only really works on attractions like Test Track or a coaster (Everest or Rock 'n Rollercoaster) where it would be common to have one or two empty seats in a vehicle based on the size of boarding parties (odd numbers in the party). Single Riders can just slip in and fill that empty seat.
On attractions like Soarin' or Flight of Passage, there are so many seats and they're all in one row its easy to match the size of the parties to fill every seat with just the normal queue.
