Here is an attempt at explaining why. Magnetic fields are additive, and by this fact, one would expect that putting two equal magnets together would double the field strength. However, magnetic field strength drops off exponentially as you move away, so the pull is at its strongest right at the surface, but significantly less so just a little ways away. Now you must consider that the two magnets do not occupy the same space. So, if you have 2 magnets stacked up and you place an object on top, you will feel the full effect of the first magnet, but will be the width of the first magnet distance away from the second magnet. Therefore the pull from the second is slightly less than the pull from the first, but still having an effect. So you have a pull that is more than a single magnet, but less than double. This continues to be the case as you add magnet stages...the third magnet is two magnet widths away, etc...
Additionally, you are still working to overcome drag and gravity forces that at some point adding magnet stages will cease to have a significant counter efect for an overall "diminishing returns" as mentioned by Keith.
Hope this helps.
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