Because with an alternate universe branching off in 2233 they've got all of Star Trek's universe and history to draw from should they wish, while not having to slavishly adhere to it.
IMO, having the entirety of Trek as a prequel (both Enterprise before and the alternate future of TNG/DS9/VOY) is awesome. What did Spock do before Romulus blew up? Watch "Unification". Who's that Archer guy? Watch Enterprise. What would Kirk have been like if his father didn't die? Watch TOS or the old movies. STXI manages to be a new beginning while still being a part of something much bigger.
IMO, having the entirety of Trek as a prequel (both Enterprise before and the alternate future of TNG/DS9/VOY) is awesome. What did Spock do before Romulus blew up? Watch "Unification". Who's that Archer guy? Watch Enterprise. What would Kirk have been like if his father didn't die? Watch TOS or the old movies. STXI manages to be a new beginning while still being a part of something much bigger.