Simple...when you use a crimp or cinch ring, you have to get the fitting in there first. With the expansion system, there's no way it will fit in there unless you expand the tubing first (and you can't wait forever to get it there, either!), since, the material's memory will have the tubing collapse onto the fitting to make the seal (only takes a few seconds). If you make the assumption (this tends to be true) that for the required strength, the wall of the fitting must be similar between the two installation methods, because the OD of an expansion fitting is larger, its ID is as well. Because the area of the opening is calculated by squaring the radius, any small difference is the square of the difference, not just the linear difference - IOW, a small difference makes a much bigger difference in the area of the opening in the fitting and its ability to flow the liquid.
So, to me, given that info, the fact that pex-A (the only one that allows expansion fittings because of the MUCH higher cross-linking in the plastic material's internal structure) is more flexible, and has a smaller minimum bend radius, it lends itself to use with fewer fittings in more locations than the competitors. Fittings tend to add up, and could offset at least some of the differences in tubing costs on a well planned installation.
You could install pex as if it were rigid copper with fittings everywhere, and I've seen some people do that. A very poor and expensive way to go...all of those fittings DO add to the friction when using water. One of the big advantages of pex over say copper is when installed with (ideally no intermediate) fittings except at each end, the friction losses mean you may not notice any flow decrease with the comparable sized copper. If you're stressing the system, you could measure it unless you went totally whacko on changes of direction with copper, but it would usually be there.
