Its because its not a season pass asmuch as its an early access as they say.
If you compare it to other games that work along a similair line.
World of warcraft, new expansion, current previous game is still there with more added BUT its a sub model. (Maybe guild wars would be a better example but their expansion are far less common).
Destiny on the other hand mirrors it perfectly. You have all the previous content, those with the base game have (although far more limited than elite does) are able to play or interact with those with the expansion, theres no monthly fee except the base game etc, it has a ten year plan, yearly one off buy etc.
The difference with those 2 though is they throw everything out at the start, HERES NEW EXPANSION, HERES LOADS OF NEW STUFF...and 3 months later, people have played it all and theres not much else to look forward to for the rest of the year except tweaks, adjustments to whats already there or minor additions etc (dont quote me, just giving examples). Elite doing it in reverse means some are less than blown away by the delivery as its all base and building plans for future stuff. Some are on a break but are waiting for 2.1, some are doing what they were doing pre horizons etc, just now that planet next to their trade station looks a bit better.
The key element is they have something to come back to over the year, every 3 or so months there "SHOULD" be something new to experiance, whether its a limp wings addition or a marmite flavoured powerplay, its atleast a large new addition. I'm willing to give them horizons to see if it'll work and for those who arnt impressed then the best time to buy horizons is next christmas when it'll be a tenner and season 3 is released.
...although it does beg the question, should this be the way to play it now theyre not the only sexy legs on the block this year?