It's the tension on the batter head and the material being used. Tension on marching snares started to increase in the mid-80's. This was to give the attack a clearer and cleaner sound. The tighter head caused a few issues such as a higher pitch (which got lost in the overall sound, or "mix") and a lower lifespan for the head. Because of this, marching snare drum heads began to made from kevlar instead of the usual mylar. This allowed drums to be "cranked-up" without becoming stretched out. But, the result was a less than natural tone. So, some marching drum lines compromised by not putting as much tension on the drum head. However, as the need to "cut-through" became more necessary and clarity became more important with more complex snare parts, the kevlar heads began to be cranked-up more and more (there are some drum lines who will use a power drill with a special drum key attachment to crank-up their kevlar drum heads). The problem with cranking up kevlar heads is that they resonate less and sound less natural. Combine that with the fact that a lot of marching snare drums have kevlar heads on the snare side of the drum, you aren't going to get much resonance or "rattle" like you would expect. It's still there, it is just harder to hear. But, again it comes down to clarity and the need to cut through the band. I don't like that sound either. On my middle school drum line's snares, we use kevlar heads, but they are not tuned so high that we loose the resonance of the drum. They also have mylar snare side heads. Yes, I have to replace those every year (where as I haven't replaced a kevlar head in about five years), but it sounds better. But, the drum line plays about 90% of the time by themselves (except for the one pep rally the band plays) so they don't have to worry about cutting through anything.
That's probably more information than you were looking for...