Because the vanilla game is meant to be still viable, and both types of players can and will meet in space, just not on the ground. Also, this is not a WoW in space. Horizons is not levelcap+10. New stuff is not required to be stronger in the first place.
And also, more generally, because it would amount to sheer power creep.
I agree with the vanilla = viable part, I disagree with the entitlement mentality that the only definition of viable = same progression, power and/or abilities as those that pay for expansion
And no, it's of course not WoW in space, nor is it D3, Guild Wars, EQ2, Elder Scrolls, etc etc - all of which follow a primary funding model of expansions with some limited in-game shop method to raise additional funds but not to the level most would consider pure pay to play while all software is free (e.g. the free to play model with primary funding being player driven purchases via an in game store)
So why don't you tell me what it IS then? Please name these other large online social games, whatever the flavor type - in space, dungeons, whatever - where expansions are sold as the primary means for the dev to continue to deliver that great game to the players - and yet somehow all the prior non-expansion buying players are on as "viable" as position as you specifically define it? As in they get the same "viable" power, progression, and/or abilities that set the expansion features apart? Please name these other common examples you must be thinking FD is somehow not delivering on.
If I can login, enjoy the game that was sold to me with the at-that-time features, that's viable. Whether those features I had are called levels, abilities, progression items whatever is immaterial - if I can continue to play as-is when I first bought that game or that past expansion - then that is viable.
Bottom line is you are insisting that because you choose to not pay for that expansion, you are not as viable as those that did. The core weakness here is your sense of what is viable or not. you are projecting the need to be equal AND ability to still play the prior core game - which you will viably be able to do whether you buy Horizons or not - but not equal to those in the new features delivered via Horizon.
Viable is free, equality is not. You are confusing the two.