Because 4 cases is not worth the negative effects of such a ban.
It was and it worked. No cases in Canada and the US was lucky with only 4... pure luck.
If countries know their citizens may face illegal and scientifically illogical travel restrictions, they are less likely to report the presence of disease outbreaks in the future.
Oh please... how can you hide a pandemic and West Africa was screaming for help during the outbreak. You're a fool if you think one of those countries can hide an ebola outbreak.