>>11297846I assume by "explore" you mean "see", considering we haven't actually explored much of space at all. Just a few satellites to the nearest planets, and only full exploration of the moon and mars, because we can actually get people and rovers there.
As for the ocean, it's easy to explain why it's not explored more: It's boring.
Now that sounds dumb on the surface, because it's not boring at all, there are such cool creatures there, but you fail to realize just how much of the ocean floor is vast expanses of nothingness. There aren't really many creatures in those areas, and the ones that are there are rarely as cool as angler fish, they are mostly microorganisms and shit. So since the seafloor has so few animals, the main people that find interest in exploring the seafloor are geological oceanographers. What do they do? Two main things: They map the ocean floor, and they take core samples to find out what types of rock are down there. Again, boring as shit. What does it being boring have to do with it? Well, you need funding in order to do more studies, and unfortunately studies that no one is interested in get less funds. And exploring the seafloor is very expensive, since you need a big ship with lots of capabilities, a bathyscape or something similar, a crew, a scanner, and so on. Who is interested in paying that much money only for the scientists to come back and say "After a month at sea, we discovered that the area we researched was basically exactly what people thought it was: a large area of nothingness.
If we're talking about why it's hard to explore it at all, you have to remember that all it takes to see planets is a telescope, so we already know a ton without even having to go there. But in the ocean once you pass where there is light, you're flying blind. We can't just light it all up and take a look, we have to send someone down there with a flashlight to look ourselves.
Source: Oceanography minor for undergrad
