Stagecoach has only operated out of Brenda Road depot since the mid 1990s (Possibly 1996), prior to that they ran from the depot in Lynn Street in Hartlepool. Before that Brenda road was home of United (Tees) and was mainly home to Bristol LHs.
Anyway, when they moved to Brenda Road, the Dennis Falcons, Leyland Lynx and Leyland Nationals had their engines mounted longitudinal under the floor at the rear of the vehicle, hence the seats went to the very rear of the bus and the engine was positioned that it was further down into the vehicle. They were just over 11 metres long rather than the full 12 metres. Buses started going to 12 metres when low floors came in, and operators not wanting to lose seats. The 39xxx MANs are either 10.8m or 11.2m, where as the 22xxx / 24xxx are either 12m or 12.8m.
They have probably tried having the long MANs and ADL Enviro 300s over the pits at Brenda Road and found it difficult to work on them, and I doubt health and safety would allow them to reverse onto the pits either, in fact I think most bus depots I've been to, vehicles tend to be driven forward onto the pits (though it also makes easier to remove the engine from a vehicle when there's nothing behind them).
But all in all, I think they've tried the idea, it didn't work hence no long single deckers at Hartlepool, no point suddenly putting 10 buses at the depot to find they cannot repair them effectively.
It applies in other cases, some depots cannot take Double Deckers, Arriva Stockton for example is one, although they can allocate double deckers to the depot, just just cannot lift them to repair them.
