Most probably, it was because of the 2001 episode, when he was alleged by a newspaper to have said that Muslims should only vote for a Muslim. Since then, Buhari has denied saying this 100 times [though at first he was slow to refute it, not bothering with newspaper allegations, as was his character] but it is already deeply etched in the psyche of non-Muslim Nigerians.
" Wakala Chama cha Siasa haruhusiwi kutangaza matokeo kwa mgombea wake wakati zoezi la kuhesbu kura likiendelea kituoni, na hili nalilisisitiza Mawakala wote ni lazima waape kiapo cha kutunza Siri, ni matakwa ya kikanuni, wakala asiyeapa na kujaza fomu asipewe ridhaa ya kuwa wakala wa chama cha siasa ndani ya kituo" Amesisitiza Bw. Kailima.