Logically, the most probable reason why progress would be slow would be if the outpost were protected in some way. If scientists had free access, progress would not be slow whatsoever, even if they didn't understand the technology behind it.
Scientists aren't dead, so there isn't a death trap surrounding the outpost. If there were, then the scientists avoided it, and so they somehow would have been able to get around it presumably. So it seems far more likely that we're talking about a barrier. An energy shield presumably takes energy to keep up, and if the alien outpost is long since abandoned, you're likely not going to find an energy shield up and running because that implies that not only is there a source of energy, that source of energy is constant and the plant converting that energy into something useable like electricity is still active without being monitored for presumably millions of years. Realistically not probable.
Therefore, my guess is that there is a physical barrier like a spherical wall protecting the outpost. That sounds rather simplistic for an advanced alien species, but consider the fact that mankind, despite having the technology to replace paper many times over, is still largely dependent on paper since making papyrus scrolls. Walls may not be interesting, but it is the most practical protection against the harshness of space and would allow an atmosphere contained within.
The aliens would have of course had a way of getting in and out, so I imagine there would be a gateway of some sort. However, how often would they need to walk on the surface of the planet? Likely not often. So it is likely that this gateway is large enough to allow the exit/entry of entire spaceships.
The reason behind the slow progress might be because the atmosphere is very much intact within the barrier. So while they could crack open the barrier:
- They don't know what type of atmopshere is underneath. It could be poisonous gas.
- The atmosphere could be under pressure, another good reason why you wouldn't want to simply break in.
- You risk contamination by the types of germs that you'd bring from earth. It could ruin any potential sensitive ecosystem under the surface.
Another reason why progress might be slow would be time interference. There might be a space-time effect surrounding the outpost that significantly slows down time for those inside. It might makes sense to do so, because ships leaving the outpost would seemingly return shortly afterwards and it means the aliens living within would prolong their lives. However it also means that spending say an hour near the outpost might be the equivalent of 8 hours to those outside it. Progress would literally be slow.