Developers can not use what ever hash algorith they feel like. All developers must use the same one or it defeats the point. SHA1 is the one agreed on by the GDF and should be used by all clients.

SHA1 was used because it would be difficult to create a fake file with the hash the matches something else.

TigerTree can be used in combination with SHA1 and would be an excellent way to provide support for the sharing of partial files.