In the first episode of Terra Nova, the 10th pilgrimage arrives in the past, and has to walk a ways to the Terra Nova colony. During the first episode, I don't recall noticing the portal terminus, but in the most recent episode, they establish that there's a terminus that fixes the arrival location of the portal. This allows pilgrimages to be easily welcomed by the Terra Nova folks. But this portal terminus is sitting out in the middle of no where. In the first episode, they had to hike back to the colony. Isn't this a huge security risk, why not just have the terminus setup inside the colony?
It's likely a combination of things. The exit point seemed to vary over different parts of land (which is why the built the exit portal to stabilize it and exit in only one spot). The portal was probably placed in a location that had an "optimal" connection with the wormhole.
You could then argue that Terra Nova should have been built around that exit, except that:
- It seems like Terra Nova construction began before the portal was built
- Terra Nova was likely built in an area that offered certain things (protection, water, power, fertile soil)
- Having a portal in the middle of a populated zone is dangerous, if something like what Lucas did with turning the portal into a weapon happened by accident, or some other thing like invasion was a possibility, it's better to put some distance between the portal and the colony.
- Perhaps to prevent sadness? Having the portal around might remind people about the people they left behind. I'm also assuming that there will be a day when the "modern" world dies off, and no one will be coming through. That would seem bad for moral to keep around.
In a word: Safety. Even though the portal was added before the 6th pilgrimage, you still have a gateway into your colony that gives anyone from 2149 who has control of the other end direct access to your colony.
Think of it this way: If you were Taylor, would you want a gate like that opening right into your colony when you can't be sure what's coming through and you don't have anything like the iris Stargate Command has on their gates?
Also, as another possibility, we don't know how wide an area the portal ranged over. From the given information of where people emerged, we don't know if the portal was able to open as far away as the colony.