Simply because the work he was doing in his eyes was not big .
Static societies can not benefit from cultural blending because they are not open to change. Such societies also are not open to new ideas.
It is beleived that the Arab pennisula was a sphere of scientific inventions. This got lost when the societies a new phase of closing to themselves. This has led to some scholars postulating that the arab pennisular stopped developing when they stopped learning from others.
He was the first European to discover America, or at least what Europe knew of. Before he discovered it Europe had no knowledge of it existing.America was discovered before by Vikings, by Leif Ericson, but their settlement didn't last that long. Columbus was the first to go to America and maintain contact with the continent and colonize it,having big impact in history. That's why it is better to say - to avoid discussions - that he re-discovered itw
