You have to remember that all bus services these days, excepting those subsidised, are run on a commercial basis. This means, of course, that larger cities will get priority over smaller towns, and loss making areas (I hate to say it, but Lincolnshire isn't exactly profitable when it comes to running a bus service).
Investment depends on return...so Stagecoach are hardly likely to go out and buy brand new coaches for the X1 when there are a load sitting spare in Scotland from Bluebird (IIRC), until they see good return on it, making investment worthwhile. This is why areas such as Darlington remain high floor Dart operated...as Stagecoach are the majority operator (Arriva are worse!), and as such can control when to cascade low floor into the town.
You have to ask yourself...what would you do in the group situation...would you a)buy 500 new buses for whereever. b)Invest more into your railway franchises (which probably make more money) or c)something completely different?
Also, when talking about Stagecoach, bear in mind that Brian Souter is on the ADL board. And that ADL have been the preferred supplier of buses to Stagecoach for many years, so much so that the only variants in their recent order are some Versas, a few Omnilinks and some Solos.
Regards
Dave