I don't think it's particularly nostalgia, or retro, land of joy or whatever term the marketing hype and media want to put on it. But, there has been a pent up demand for this type of bike for some time, whilst manufacturers chased higher perfromance and greater technology, resulting in many riders just feeling a bit cold about the latest and greatest offerings.
Most everyday riders realise that they do not have the skill (or even desire) to ride a 180 bhp bike at anywhere near it's limit, and if they do there are serious risks involved in doing so safely on public roads. To ride such bikes at more realistic day to day speeds is a bit of a flat experience, you're not so much a rider as a passenger.
Enter bikes such as the Scrambler, light punchy, quick, loads of fun to ride, no techno wizardry, just a plain, old school, no frills honest bike. Equally fun to ride if your a newbie or a long term experienced rider. A simple blank canvas which you can make into your own version of perfect and enjoy doing it.
Triumph have been sellingg such a bike for 15 years now in the Bonneville, and like Ducati's Scrambler, the various Bonnie's are Triumphs best sellers, whilst being far from their best bikes. Heavier and slower than the Scrambler, I wouldn't want to sell mine, but ride the Scrambler and it definitely feels like the new, young kid on the block.
BMW made a cock up with the Nine-T in my opinion, too complicated, too confused and too expensive. It'll be interesting to see if they can regain some ground with the new range of 'basic' bikes that's mooted to be coming for 2016.
Meanwhile, Ducati seem to have re-found the formula that made the original Monster such a hit, and long may it continue.