Single deckers are universal throughout Europe (and even wider), whereas double-deckers are not universally popular.

The B9TL evolved from the Olympian (Leyland and then the Volvo version).
Whilst (to us) it makes sense to perpetuate the B9TL, Volvo might be taking a completely different view.
We might not like it, but it's their train-set.

Reasons might be financial (profitability) or rationalisation (what percentage of their market does the double-decker represent?).

http://www.route-one.net/vehicles/test-d...evolution/

The B8RLE is obviously a completely new chassis (though based on earlier models) which has had to go through testing at Millbrook. The results of this testing would be adequate for endorsing this model for 'Europe'.

A new double decker chassis would need testing for durability - and, as stated above, the market for double deckers is largely restricted to the UK (and, perhaps, London in particular - I don't know relative volumes).

What makes and models are being introduced in Hong Kong (another market for double deckers)?