Because that is the timeline they believe it will take.
Go to a restaurant and they tell you your meal will take 15 minutes. I'm sure some will ask why 15 minutes....I'm because that's how long it takes to do it
Hmm maybe as this will be the first non 1D body to be able to focus at f8' Perhaps it is a big deal to code the firmware. Or my own theory best to make a big deal about it, or there may be a big demand for all non 1D bodies could be done as well..