Pope Francis is being accused of covering up sex abuse in the church. If true he is the same as all the rest. It not why would someone lie?

You're right.  If it is true, he is no different than the ones who committed these acts, and should share in their punishment. 

People lie for different reasons.  For example, some lie to avoid embarrassment (I did not have relations with Ms. Lewinsky).  Some lie to be polite (no, honey, you do not look fat in those jeans).  Some lie to discredit others (Martha's easy) .  Some lie just for attention (I caught a fish that was at least the size of Texas).  

If it isn't true, perhaps the one who lied di dso to discredit Mr. Francis.