Why would a LED datasheet specify a minimum operating current?
To be honest, I don't really know.

But they also give this graph:



It may be that under 20mA the chromacity shift is much higher and the colour of the LED changes noticeably. As the other parts of the datasheet stress on CRI (Colour Reproduction Index) qualities, it may be that the LED wouldn't fit into the specs @ &lt;20 mA.