Why should it be. Every survival run thing so far was a simulation within the Infinite Forest. The introduction quests even stated that in the text. AFAIK the Vex never conquered Earth, therefore there can't be a direct portal in the EDZ patrol zone. They only put in an activity start icon in the tower because they couldn't implement it on Mercury, which was a DLC location. And non-DLC players had no access to Mercury. Dark Forest was an event, therefore it had to be made available for all players.