Could anyone explain why differential biasing is easier than single-ended one?
A differential amplifier usually has a tail-current source that sets the bias current of the transistors of the differential pair. 

It is usually used in a feedback configuration, thus it can control its own operating point taking advantage of the feedback.

Differential amplifiers are often used in integrated circuits where some kind of reference current is available, that can be replicated using current mirrors. It is thereby possible to set the bias current accurately and determine the operating quite accurately.