Think it's down to Wenger trusting in his 'old guard' CB pairing of Per and Laurent. He did arrive after they both struck a strong stewardship between them right? Ideally, it's down to him to have the cojones to take his chances when either of them is plagued with an injury. Last year he looked unassertive and shaky somewhat to be fair, however I do really rate him. Just feel he needs time to bed in when selected constantly.
Holding may have been bought to counter this incase Calum's path goes tits up elsewhere. Watford is a jump over the bush for him, so he won't be unsettled with moving etc. Makes sense. Bellerin benefitted from his loan there.