Gravel Riding: Why gravel wheel over lighter XC mtb wheel?

Since the question was asked, high-performance carbon wheels for drop bar bikes are evolving towards an internal width of 22-25mm - that is, wheels aimed at both performance road and gravel bikes. I am not sure how alloy wheels for drop bar bikes are evolving, but I suspect they will follow (White Industries already makes an alloy rim with a 25mm internal width, and there may be a few other small brands that I can't remember). I believe this would have been considered wide even for mountain bikes in the early 2010s. That said, while I'm not familiar with MTB disciplines these days, I think at MTB wheels are approaching 30mm internal width.

I can't see any substantive reason not to use an XC wheelset on a gravel bike if one is available, provided that the spacing matches the bike. I believe current MTBs use the Boost standard, i.e. the front and rear dropout spacing is 110mm and 148mm respectively. On drop bar bikes, this doesn't yet give an advantage, and as far as I know the majority of gravel bikes and all road bikes have converged on 100/142mm spacing. I'm not sure what the state of play for Boost spacing was in 2017, but it is an issue now.

The folks at the blog Riding Gravel noted that you can often get good deals on second hand, high-end 29er, non-Boost 29er wheels, as this is a dated standard on MTBs. Because many of the front hubs used 15mm thru axles and most drop bar bikes use 12mm, you often have to source some sort of adapter. These wheels will also tend to have 6 bolt disc rotor mounts, whereas the drop bar world uses center lock mounts. One would simply acquire a 6 bolt rotor and use that.

While aerodynamics and gravel might not have been as big a deal in 2017, I suspect this trend will develop for gravel bikes. If you used an XC wheelset, you might make a minor sacrifice versus a deeper section road-oriented wheelset. This sacrifice won't be relevant to all users.

As to the specific wheels mentioned in the original question, there may have been no substantive reason to choose one or the other on a gravel bike. So, if you could buy the Crossmax Pro for less than the Allroad Pro back then and you were willing to use a 6-bolt rotor, then you should probably have done so (keeping in mind you needed an adapter for a 12mm thru axle; Mavic doesn't supply this with the current Crossmax Pro linked by Cray Kao).