Because Aarnuld is an old man.
In terms of this film though, going by the trailer, Skynet has probably sent another terminator to an earlier point in Sarah Conner's timeline, leading to another 101 being sent back to protect her, which has then stayed with her and continued protecting her, aging over the years as it did so.
They should have ditched Aarnld, but they wont make it without him because of the money involved. Its perfectly reasonable to assume that there was more than one 101, having them all look like sausage fingers wouldn't make for very good infiltration units "hey he looks exactly like that German terminator we blew up yesterday, shoot him!"
They should have got the bloke who played the mountain in game of thrones in to play the new 101 and bin the Aarnuld model once and for all.
The fact that Arnold is in it suggests that they wrote the script entirely around the fact that he is now old, and so they had to come up with some bullshit story involving an alternate timeline, and that in itself will lead to the inevitable paradox's and plot holes in the story.
For example, going by the trailer, Reese doesn't know anything about a T-800 being sent back further in time from the first timeline in T1 to protect a very young Sarah Conner. So, by that logic, the future would have changed, and everyone in that future would know about that alternate timeline; therefore, the events of T1 and T2 would not have taken place like they did, so Reese would have already known all of this based on his future timeline. So, if the events of T1 and T2 did not take place, then why would they send Reese back to save her from the first T-800 from T1?
I don't even know why they feel the need to "re-do" everything by keeping the time travel element in there and setting it pre-war. We've had all that already. This is just Hollywood tying their best to reboot something again, and then coming up with another two needless sequels, because the war of the machines will still fucking happen. It's infuriating that they do this. We've already seen them do that recently with Spiderman and X-Men, and there's going to be another two sequels of those.
