There's a simple reason that nylon strings don't sound as 'treble' as steel; they have a lot less high frequency energy. It's simply the nature of the strings.
As a material nylon has much higher 'damping' than steel. That is, if you tap on a plate of nylon it goes 'thud', while steel tends to ring for a while. The nylon actually dissipates the energy of vibration faster than steel. This effect is more pronounced as you go higher in frequency.
Also, because nylon is much less dense than steel nylon strings need to be thicker to carry similar tension at a given pitch. The thicker strings have to move more air as they vibrate: it's sort of like trying to run in knee deep water. This also tends to cut down on high frequencies fast.
You can use something like a UST to actually see the force the string puts on the top of the bridge saddle as it vibrates. If you set one up on a stiff and heavy beam you can look at the string itself without the vibrations of the body changing things. If you pluck a steel string and a nylon string on such a rig you'll see that the initial wave form will be exactly the same for both. They will have the same amount of energy in each overtone to begin with, assuming the plucks are the same strength and in the same place. Within a second or so the nylon string will have very little energy above about 3000-4000 Hz, while the steel string will still be going strong out to at least 8000.
This is why steel string guitars tend to be made differently from nylon string ones, aside from the generally higher tension of steel. On a steel string instrument you have to get enough bass to balance out all of the high end. With nylon strings you have to get the most out of the small amount of high frequency the strings are giving you. Many of the differences in design and construction follow from that. Its also why it is often less than satisfactory to swap string types on a given guitar.
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