Huh? Motivations unknown to the public? Holy crap, then you are uneducated....
The physics and chemistry prizes are awarded by Kungliga Vetenskapsakademien(Royal Academy of Science), whose everyday task is to promote science. In accordance with the rules laid down in the will, they are tasked with promoting science that leads to advancement for mankind. Thus, by necessity, they promote science that leads to practical advancements and not just "pure" theoretical advancements.
The Nobel Prize for Physiology/Medicine is awarded by Karolinska Sjukhuset(A fairly renowned hospital with a significant research and education division). As above, their task, as laid down in the will, is to promote science by rewarding practical progress that leads to the betterment of mankind, and not just "pure" theoretical research.
The Nobel Prize in Litterature is awarded by Svenska Akademien, whose task in awarding the Nobel Prize is by following the rules of the will, which is in fact somewhat problematic, because if they were to strictly follow the rules, they'd no longer be able to hand out any prize at all, due to how litterary styles and tastes have changed.
The Nobel Peace Prize is handed out by the Norwegian Nobel Committe, which is selected by the Norwegian Parliament, according to the rules set out in the will.