Why aren't sportive' 'bikes popular outside the UK?
That site's basic definition of sportive boils down to:


more upright riding position
more comfortable for distance riding
longer chainstays, taller head tube
softer forks (though many brands have gone to carbon forks across the board)
yet still with drop handlebars and quality components


And I think nearly every brand in the US has entries in that realm, though they often call them things like "endurance / sport / performance" as compared to "race / competition". Basically it comes down to "comfort geometry not race geometry"; components, materials, and even gearing choices are often similar or identical.   Of course their race models often extend up into exotic carbon frames and top-end component groups, where sport models may only go up to basic carbon frames and mid-level groupos.

Some examples (aluminum frames):


Brand          sport geometry     race geometry
-----          --------------     -------------
Cannondale     Synapse            CAAD 10
Felt           Z series           F series
Giant          Defy               TCR 
Specialized    Secteur            Allez
Trek           ?                  2 series


These are all drop-bar road bikes, but the first column has a little more forgiving ride for long distances, while the last column has racier / faster handling.  But both kinds are raceable if you want (they're not all that far apart).  Both can usually be had with your choice of triple or compact double crank gears; a standard double seems more available in race models.

So 'sportive' bikes are available in the US, just by different names.