Plainly and simply, "the track" as the 110 has long been known by drivers (IIRC) is one of the busiest - and presumably most profitable - routes in West Yorkshire and has long since been, and for many, many years has been able to justify regular new vehicles that ensure cascading of nearly-new vehicles to other routes.
It benefits from a direct, relatively fast route between Wakefield and Leeds (even during the rush hour it is timetabled to take just 40 minutes between the two cities), and being so frequent it will always encourage more use - not to mention the long-standing poor perception of local railway services in West Yorkshire (something that the 36 has, of course, benefitted from enormously, too).
The other thing to consider, though - Leeds is the economic engine of the entire region, and with the decline of industry in the towns and cities that ring Leeds, many of the other places locally have basically become dormitory towns for Leeds. Huddersfield is still able to command six or seven trains per hour to Leeds - the longer distance and slower services, while the trains are fast and direct, meaning that the buses don't quite benefit as much (but even the 202/203 remain a combined x15min frequency), while Bradford, Wakefield, Harrogate, Keighley, Shipley, Castleford/Normanton/Pontefract and Wetherby all have frequent and fast services into Leeds that are very well used and - Arriva aside right now, perhaps - have had good investment in the vehicles to up the quality that commuters demand.