You can find the resignation letter here. Commissioner Martinez explains:

I am leaving the EAC strictly for personal and family considerations. My wife, Beth, and I have been blessed with two wonderful children – Sofia Grace and Lorenzo Elder – and we have decided that it is in the best interest of our family to return home to Austin, Texas and focus on our future. Moreover, the recent and unexpected death of my mother has reinforced our desire to be closer to our immediate family.

I completely understand the sentiment; I have made a number of job decisions for family reasons. And I wish the Commissioner well.
But this is a real blow for the cause of election reform. I have gotten to know Commissioner Martinez over the last year, and his service on the EAC has been exemplary. Though he was appointed as a Democrat, he served the office committed to the ideal of nonpartisanship (indeed, he has a forthcoming piece in the Election Law Journal urging that chief election officers in each state take concrete steps to assure that they administer their offices in a nonpartisan manner). He is a lawyer who has participated in a number of academic events, mostly there to listen and learn, but also willing to engage in frank and honest discussion. He is a real leader, and he will be missed by those who care about serious election reform.