I assume the sequential detonations just exist so we can leave a trail of C4, leading from the farthest point we're keeping an eye on towards our current position, detonate the first (ideally furthest) one when a cluster of zeds is near it.. and then have the next one immediately ready for the next group of zeds (or any tough ones that survive the first blast)
But with the major advantage over the proximity sensors of being able to choose NOT to detonate, if unfavorable enemies are crossing (small groups, scrakes, whatever)
Having a "detonate all" button might not be necessary if explosives, including other C4, will set these things off.. we don't know one way or another yet. But given how cheap and effortless the "huge pile of pipe-bombs" strategy made defeating Patty in the first game, and the steps necessary to nerf the device because of it, it could be possible that Tripwire just doesn't want that sort of thing done any more.
As for detonating whichever C4 charge we look at.. that seems kind of redundant when ALL THREE of our other weapons have the sole purpose of "explode the thing I'm pointing at", don'tcha think? Besides, without magical UI elements are you going to be able to remember exactly where you put each of your C4 charges and which ones you've already detonated? (okay I guess the scorch marks and even coating of blood will be a good indication.... the first time around) When will this even be practically useful, in the typical Killing Floor scenario? You're going to simultaneously keep a constant watch on three or four different choke points to look for opportunities to detonate the right charge at the right time, AND actually help fight the zeds? Even if you did it by putting them behind sealed doors and waiting until you hear one get broken down, you could only do that ONCE per door.
Plus that ability would interfere with the sequential detonation (when you have multiple C4 charges in front of you at the same time), which would mean NOT being able to set up multiple points of explosive defense along the same path. Logistically, I think we're going to find THAT viable far more often than trying to camp multiple points.