One reason airlines may specify a particular TRTO for a type rating is that they have agreed with them to reach the rating using their own SOPs, therefore transfer to line flying is simpler. Making such an arrangement with a large number of TRTOs may not be worth the bother to them. Much easier for them to specify one or two TRTOs than have numerous meetings with a lot of TRTOs discussing individual training requirements. Maybe not fair but airlines will always chose the easiest route where possible.