Why is it difficult to find evidence of life on other planets

Due to limited knowledge and strength to explore the universe

Explanation:

As an human being our potential is limited to only accessing the things which we can see or imagine or about which we have studied in our schools and colleges

Apart from life forms on planet earth,  humans are not aware of any other life form on any other planet. Nor they have any idea whether the life form on other planets will have the same characteristic as that of life forms on earth or it may be exceptionally a different scenario.

We have not yet explored universe beyond our solar system and hence we have very very limited knowledge scope.

Thus, it is difficult to find evidence of life on other planets