That's because this is precisely what they did. And don't get me wrong, i understand why, RO2 is on a much bigger budget, and thus, it also needs to sell more copies to make a profit, that's basic economics 101, and as such i have no objections to them including the arcade mode, Developers need to eat and pay bills too.
What i don't understand, however, is why they didn't also include a really-real Realism mode for all their realism fans. And no, the mode that is called "Realism" isen't that, it doesen't cut the mustard, but they easilly could include a 3'rd official mode for the realism nerds amongst us, and only stand to gain even more happy players doing so.
But they haven't been interested in doing that for whatever reason, and that doesen't make sense to me, because i think they would be selling more copies if they did, lots of the old RO guys would love such a thing, and as would players from franchises like the ArmA, WWIIOL and America's Army series, so why not bait a fishing hook for thouse players aswell?