World exploration by Europe

Hi.

Why is it that the vast majority of world exploration by Europe
happened in relatively modern times (1500s to 1600s), and not further
back in the past? Not enough technology? Not enough motivation? It
seems there was 1400 years since Ptolemy published his map,
summarizing the European geographical knowledge of the time, which
covered only around 1/4 of the planet, to the time when new
exploration was begun, and the remaining 3/4 of the globe was figured
out, and it _seems_ that geographical knowledge was pretty much
stagnant throughout that time. For example, why did it have to wait
until Marco Polo in order for the existence of Japan to have come to
be known by Europeans? Japan is not all that far off China, and in
ancient times the Silk Route enabled trade with the Far East, so even
if they hadn't seen it directly they nevertheless could have heard
about it. Why didn't they? Why doesn't "Japan" turn up in Western
texts before Marco Polo's trip? And what of the Pacific Ocean? Why did
Balboa have to go to America to "discover" it when it could have been
discovered long before by simply traveling to the Orient and seeing
the big ocean off the coast?