Both. Many natives are unemployed because reservations were placed in really inconvenient and resource-poor places. Check out the employment rates in neighboring areas: also low.
There's also been a huge transfer of land from tribal to non-tribal/non-Indian ownership [1]: the US government held a lot of land in trust for the tribes, identified a lot of it as "surplus to their needs," and sold it to non-tribal members, and on other reservations they transferred ownership to individual tribal members and then economic pressures pushed them to sell, especially if they traditionally did agriculture and the land wasn't farmable. This land wasn't developed, in general, so there's really not that many places to work on a reservation...
There's also been a huge transfer of land from tribal to non-tribal/non-Indian ownership [1]: the US government held a lot of land in trust for the tribes, identified a lot of it as "surplus to their needs," and sold it to non-tribal members, and on other reservations they transferred ownership to individual tribal members and then economic pressures pushed them to sell, especially if they traditionally did agriculture and the land wasn't farmable. This land wasn't developed, in general, so there's really not that many places to work on a reservation...
[1] https://www.iltf.org/resources/land-tenure-history/allotment