The only reason that i can think of is that there will be more load in these columns as they see more roof area. Thus if the roof bracing and cross braing are in the second last bay, then there will be some more downward load to counteract the vertical uplift component of the axial force in the cross bracing.
The main reason is that the roof bracing connections are easier when you have reafters top and bottom acting as the chords.
Also your end walls are not always frames and they are therefore often smaller members to take the chord compression force.
Another reason is construction, The erector should start at the braced bay, erect the bracing and move on from there. If this was at a non portal frame end bay then you would have a lot more members to put up before the structure was braced.
Like csd, I also believe it has to do with the economy of using a member that happens to already be large (interior bay) to carry the chord compressive force rather than having to upsize a smaller member in the exterior bay.
Some endwall rafters (at non-expandable endwalls) can be made of old formed members, which may not have the resistance to keep the bracing from pulling through the webs.
