If you examine those who have failed alongside those who have succeeded, and are unable to find an explanatory variable, related to personal contribution, that

If you examine those who have failed alongside those who have succeeded, and are unable to find an explanatory variable, related to personal contribution, that cleanly distinguishes the two groups, then you might conclude that external factors (i.e. "luck") have a role to play in success. If you don't examine the failures then you can't determine if this is the case, and you might fall foul to the logical error of survivorship bias, as has already been mentioned. edit: changed "an important role" to "a role", given that lack of a "clean" distinction could be of any magnitude.