Why can't hubble see planet 9 if it can see distant galaxies?

Two thoughts come to mind. Up to now the location of any such planet hasn't been confirmed and needless to say the HST has a very small field of view and so needs to be pointed very accurately. If astronomers do determine (by calculation) where the planet would be located in the sky then the HST would show it as a small, almost featureless disc.

Secondly as a previous answer has also mentioned, galaxies contain some hundreds of billions of stars and as such are much more luminous than any single star or planet could ever be. The human eye on its own can discern the next nearest galaxy to the Milky Way at 2.5 million light years distance but you need a large telescope to see Pluto at just 40AUs distance.

First, where do you look?
Second, galaxies are large and emit EMR.
In a dark room even a lit match is easier to see than a sofa.

Hubble probably could see it but the problem is pointing it in the right direction. Furthermore, if the planet is as big as the Cal-Tech team says, it should be visible to some of the largest Earth based telescopes at aphelion, at perihelion it should be visible even to medium sized telescopes on Earth so for most of its orbit (which is about 10-20,000 years) it should be visible to many telescopes, so it's just a question of refining the maths and narrowing down where to look.

Bear in mind it won't be found by just pointing a telescope and seeing something that looks like that artists impression. It will just look like a normal star, just a point of light. It will only be detected by carefully comparing the images taken several months or years apart. Remember if it takes 10-20,000 years to orbit the Sun then it moves very slowly and so we will either have to wait a long time or hope that it shows up on some old photos.

We don't know where it is (if it exists)
The 8 planets all orbit in more or less the same plane.
Planet Nine supposedly orbits in a different plane. Therefore it could be anywhere in the sky.
And it would be very faint.
And it would be moving very slowly.

We are photographing the entire sky at higher and higher magnification. So computers can be programmed to compare photos of the same area - looking for faint objects that move. Then each of those would need to be investigated.

Note that when someone "finds" a new comet, asteroid or Kuiper Belt Object, there is usually a previous "sighting" that can be seen on photographs taken before the discovery.

Galaxies are very big and very bright. Compared to them, planet 9 is very very small and very very dim.

Distant Galaxies glow.
Planets reflect light.
A planet out there is in almost complete darkness.
Maybe if it put its headlights on.

To see it we need to know exactly where to look.

Distant galaxies are 100's of thousands of light years across and emit light..... Unlike a tiny planet that is billions and billions of miles away from a star and produces no light of its own..... Even then, it would be able to see it. But ya know....nobody has any clue where this planet might actually be at ya know. Where do you propose pointing Hubble?

Apart from the other answers, it's existence is still speculative.

Galaxies are much, much bigger than a mere planet. That 100,000x9.5 TRILLION km vs 10,000 km.

You have to know where to look first.

Becasue you have to point Hubble at it, BUT NO ON KNOWS WHERE IT IS.

They HAVE NOT yet found an actual planet, only "evidence" that one exists. But this "evidence" COULD be caused by another source, something that IS NOT a planet.

So far, "Planet 9" is still JUST A THEORY

It most likely can, we just need to know where it is exactly.

Because no-one has set it to look in the right place. Space is big.

It is really far away and does not reflect much light, and it's position can only be generally inferred. It is like a needle in a haystack the size of Himalayas, or something like that. Maybe Hubble will find it though, I am sure every astronomer is looking hard. It is probably an alien base wormhole device used to stage an invasion until they found a better planet that was not so polluted and abandoned it.

Stars shine, planets do not, only reflect some starlight. Hard to see.... See?

The only way you can tell the difference between a fixed star and a planet is movement. A ninth planet may have already been detected but passed over as a fixed star, it takes a sequence of images to determine motion and if no one has done that we would never know.
Galileo actually was the first person to discover Neptune but he made the mistake of believing it to be a fixed star , it took another two centuries before it was determined to be a planet .

Galaxies are stationary and even still it takes up to 20 days of exposures to get the image that you need. Planets are moving and a long exposure cannot be done

There's a reasonable chance that it (or some other big 'scope) already has. It will look like a faint star so there's no reason to single it out. Right now people are trawling through the star surveys looking for it.

Hubble nor any other bubble can't see it, because there is no "planet 9".
Nearly two thousand planets (not luminous stars, nor galaxies) have been discovered in the past two decades; many of them have been "imaged" & you can see the pictures of them. With these technologies you can see rocks floating in the outer reaches of Oort-Opik cloud. But there are billions of them & nobody cares to track them. So it is not impossible to see a planet lurking somewhere in less than ⅓ light year distance, when we are already "seeing" images of good sized planets a couple thousand light years away.
Is there any credibility in such a statement as you made or imply?