The reason why is because Xbox will be left in the dust
Halo is Microsoft's flagship series that has been selling its consoles for 18 years; that isn't something they'll abandon any time soon, regardless of its exclusivity status. If they support Halo they'll support it on their own console, perhaps even more than on PC.
Xbox players will only be shafted if new titles in the series are poorly received (which will affect both platforms) or the consoles become ridiculously unpopular to the point that there is no playerbase; in both cases there are greater issues to worry about.
An argument could be made that a lack of crossplay would detract from the Xbox playerbase but considering 343's reluctance to tweak controller aim assist I expect we'll see it implemented before too long.
I agree with the above by DC 409871
- if Xbox is in a bad enough spot that this is indeed an issue, implementing map exclusivity is trying to extinguishing a house fire with a spray bottle.