A valid reason would be the scarcity of resources. Further technological progress will be severely constrained by which chemical elements are available cheaply and which are not. Lots of interesting and useful chemical elements are not available in sufficiently concentrated ores, or they are rare in all of earth's crust, having sunken down inside earth's core during its formation.
These elements thusly are produced only as by-products of other elements which are more concentrated in their ores. This is valid not only for most of the lanthanides, but also for elements like indium, tellurium, gallium, germanium and the platinum group metals.
Asteroids might hold rich deposits of these elements because the elements could not sink down into their cores, and even if they did, most asteroids are small enough.
So if we don't want to substitute that indium tin oxide in our smartphone touchscreens with cheaper elements, we'll have to mine asteroids.
Edit²: Here's a relevant review article (Vesborg, Jaramillo 2012)
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