Why?
Because 4 months is the estimated shelf life they've projected for their title. After that, Ubisoft will pursue sales in the secondary market, which happens to be the PC market.
I imagine part of it has to do with Ubisoft wanting to minimize the effects of piracy (it's less common, though obviously not impossible, to pirate software on consoles than on a PC) until it absolutely has to.
Think of it from the marketer's perspective, which sounds more appealing for your bottom line:
Release a product that will be cracked, copied and readily distributed immediately, thus cutting into your sales for more consistent markets.
Release that product later, so that said people cannot immediately crack and distribute it (or if they do, it will be distributed for a much smaller black market population).