What would happen if we put Swallow in and he got injured?
Drew is 29. By the time his 3 years is up he will 32. Swallow will be 28 and Jack will be 24. Do we then give Swallow 3 years as Captain because he deserves it? Jack would be 27 by then and we have possibly kept the best leader at the club out of a job for 6 years because we are giving it to blokes who "deserve" it by longevity as opposed to leadership qualities.
I suppose gut feeling is that Ziebell would be the better captain, but he hasn't really played a full season yet.
Drew is a leader & if you put him in then there is no chance of offence to Swallow. Then when Drew finishes up, Ziebell is 24 and more able to take on the role. Less risky than putting Z in now & seeing him break his 3rd leg this year.