Thanks in part to his corruption (as well as his poor performance against Boko Haram), Jonathan lost to Buhari, and now Buhari seems to be delivering on his promise to root out corruption. "Mr Buhari has sacked the NNPC’s board and earlier this month named Emmanuel Kachikwu, a Harvard-trained lawyer and former Exxon Mobil bigwig, as chief executive. He has already started axing old-guard executives."
It will take more than that to solve Nigeria's corruption problems (Nigeria isn't at the bottom of Transparency International's ranking any more but it's fairly close - tied with Russia and a few other countries at #136 out of 174 countries). But it's a good start.