Challenge 1:

Players would feel comfortable playing exclusively in PvE. This would mean that they would want to just consume the PvE content, and once they are done with that why would they stick around. Unless PvE is structured to also allow for some PvP then this 'consumption rate' would make development time in it not very efficient.

Challenge 2:

The amount of content in the PvE side of the game would have to accommodate SP progression like PvP does. There would need to be entry level content that can be done in blueprint suits, then there would have to be content that maybe takes multiple squads with 4 HAVs and dropships etc and strong coordination with high skilled players. All the while, the rewards would have to be variable enough, but not totally random to make the significant risk worth the fight.

Challenge 3:

When your player base is as small as it is, making a new and enticing whole additional 'game mode' would divide the player base even more. More players in PvE means less players queing which likely ends up with more uneven matches. This results in worse player retention and a shrinking base. Additionally, if players spend a good early portion of their time getting SP through PvE, they wouldn't necessarily be skilled for PvP.

A big chunk of PvE would be, "Does it come before or after a player market?"

If it is before a player market, then whatever its output is, would have to be somehow useful for doing more PvE and or PvP. The devs have said that they wouldn't want PvE to be required to be a grind for everyone in order to compete in PvP. If its ouput was just isk that wouldn't be very interesting as a reward. It would need something unique and worth risking costs similar to pub matches. There are some pretty long threads talking about these options so I wont bother.

If it came after a player market, then the way its rewards would be useful would be a little bit easier and doesn't need explanation beyond, "You would just sell what you find."