This has been asked and discussed a number of times in this subreddit. See for example [this] (https://www.reddit.com/r/AskHistorians/comments/3h7svk/what_factors_led_to_ethiopia_being_successful_in/) and [this] (https://www.reddit.com/r/AskHistorians/comments/1ddde1/ethiopia_stayed_independent_during_the_19th20th_c/). It's always possible there's a new answer out there.

For a more complete analysis, you could always read Raymond James' The Battle of Adwa*. Or for something more fun, watch Teddy Afro's music video answer, [Tikur Sew] (https://www.youtube.com/watch?v=IuyfK7NLosY)