IMHO the decision was influenced by:
Moore - good (incumbent) captain but reduced time on field by his own desire to reduce workload, introduction of new hooker combination and his departure at the end of the season. (his departure would influence the psychology of the playing group, its human nature; like working with a contractor - in your mind you know once he has done his bit he is gone so his commitment is always questionable both to quality of the outcome and anything bigger picture).
Lealiifano - Stable, consistent and well entrenched in the club. Communicates well and has shown leadership abilities including mentoring the younger players. Organises and lead well on field and plays most games out.
Others like Pocock and Fardy would be tempting. I think Pocock if re-signed is a front runner, and Fardy is almost the unofficial on field captain.
It sounds odd but if you watch Fardy he has the ability to be able to engage with nearly all the referees on field which is quite unique and a real benefit. If you watch referees, at times they will seek him out to provided him with explanations etc. Its an asset to any team and almost worth more than having him as a captain.