I think you're making comparison with how timeline is being presented in real life.
You could ask the same question why Facebook, Google+, or any other timeline based apps, present the latest event at the top and past events below.
First and foremost, in designing how information is being presented to the user, we need to understand what the user is most interested in. In such apps, our user is most interested with latest event, which would then be presented first in the list, be it horizontal or vertical. Past events are usually of second importance level, which would then be listed in reverse chronological order after the first in the list.
Reading top-down and left-right are the most natural way (except for RTL languages). So you'd then present your information from latest to oldest in top-down or left-right manner.
You can definitely refer to how Windows 8 apps present information horizontally.
On a side note, when looking at historical timeline, you're usually more interested to find out how something started and got to the way it is/was eventually. Hence, oldest to latest in left-right or top-bottom manner.
Hope that answers your question.