Plot element

If all else fails, there is always "Unobtanium".
[which you may define].

But, does it have to be an economic reason?

[Is it or does it [at first] seem necessary that they return?]

They may have a cultural need to isolated, or they may be
running away from something e.g.,-a disease without a cure,
religious, racial, or cultural persecution, -the establishment of
an "Evil Empire", etc.

RKH