Simply put, because at lengths below this interactions are mostly governed by QM. It is important to be aware that this is not an absolute limit.
Small addition. Some experiments in deformation and fracture of metals and alloys have shown that mechanical properties of material could be dramatically changed when the grain size becomes less than this limit.
first, "100 nm" is kind of arbitrarily taken, there is no clear distinct boundary; one could also have taken 127 or 86 nm, for the same reasons, but as we all like round numbers, it is better to take "orders of magnitude". So it is not "100 nm", but "10^2 nm" which is taken as the boundary below which we talk about nanomaterials.
The major reason is that from this size order of magnitude on, the surface plays a bigger role for the properties and performance than the bulk. The smaller the particles, the higher the percentage of atoms / molecules which are located at the surface, hence do not have the same environment as atoms / molecules in the bulk.
Moreover, surfaces as such are taking over the decisive role for many properties of the nanomaterial and make them different from their macroscopic bulk material.
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