Cyclocross bikes used to be the reserve of a small band of devotees and "serious" cyclists looking to train and race while there was salt on the roads. In Europe they predated mountain bikes of course as a form of racing, but were usually regarded as a quirky investment.
Now the wheel so to speak, has come full circle again after the long love affair with the 26" wheel, 1.95 inch tyre and straight handle bars wanes. Where once riding a muddy fox courier was cool and a sign of "hipster" rebellion, now mountain bikes are standard consumer items for the masses. Teenagers bikes today would have been exotic items to drool over in the 1990s. Cyclist "nerds" were looking for something more exclusive and esoteric, and therefore the explosive rebirth of cyclocross as a sport began.
Just to focus on the bikes: there is a happy sweet point now with technology and price point. Now you can have disc brakes which give extra stopping power and much longer life to rims. Frames are quite light at the entry level (which is about mid level compared to mountain or road bikes) but importantly they are stiff with good clearances. And then most of all there are the shifters: gone are the drop bar end ungainly shift of old cyclocross, in are hood shift-brake units from road bikes.
The next move will be to use the mid sized 584mm wheels which are half way between mountain bike and 700c road clincher wheels. This will make the bikes more sturdy, and offer the average user spoke key free seasons on a wide set of terrain.
What i liked about hybrids was their lack of pretention: they were city street bikes as much as they were "trail" bikes. I worked in bikeshops when there was a steady stream of people wanting to ahem, drop the drops so they could commute with better access to brakes and gears in the morning rush hour traffic. Mountain bikes, like range rovers, offer a tough off road pretence while most users will spend most of their lives on tarmac.
In my days of youth though, i loved mountain bikes because you could just go mad down a rooted, bolder topped path and then ride back to town. You could power slide corners and sprint downhill. However the older me wants to do longer sessions now and explore the countryside more, joining up forrestry roads, tarmac and the odd dry good footpath to make for a 2-4 hour ride.
Drop handle bars have such a big windage and comfort variation factor that they are a must for faster and longer runs. The " African" tour bikes of the 1990s were very often either hand made hybrid on 26" wheels with drops, or just bastardised MTBs with bar end or the legendery Deore top bar thumb shifters so loved by serious mountain bikers in the early nineties over the plasticy double lever under bar cousins of the time.
That is another thing these days of getting three bikes in one wijth a cyclocross: trail and cross bike for smoother off roading, touring bike for paniers and road racer with 23C tyres at 120psi.
Most of my riding now then would be better on a cyclocross bike. However i do enjoy unlocking my forks for a comfortable blast down hill or a trip off the beaten track.
There is my tuppence on the whole MTB vs Cyclocross debate.