Why would demanding that Indians own private property make them more vulnerable to losing their land as opposed to if they continued to use the land in common?

When you own something, you control the resources. You control that thing, and someone else may want it. Because you're the only one protecting it, you're weaker.

When you share something, however, that thing isn't controlled by one single group of people. There is no real reason to claim land that you already have safe access to. Plus, anyone else who is a part of that land will also protect it from enemies. As has been said a million times and will be said a million more: there is safety in numbers.

If you demand Indians to own their own property they will lose land because since we have many new technologies they will not be able to use these technologies properly and some people might force them to use the technologies. If they used the land commonly they will use resources they used before and no one will tell them what to use.