I think a lot of folks see these gorgeous photos of distant galaxies, with fine detail on dust lanes and spiral arms and assume that since they’re so far away, seeing Pluto would be easy. But while these galaxies are far away, they’re also huge.
The (relative) detail that can be seen in any given telescope will be found by the object’s size divided by the distance from the telescope. In other words, the angular size in the sky.
$$\theta = \frac{\mathrm{size}}{\mathrm{distance}}$$
Let’s compare the angular size of a distant galaxy with Pluto. A nice spiral galaxy (like ours) is about 100,000 light years in extent. We’ll make it old and put it at 10 billion light years away. How big is it in a telescope?
$$\theta = \frac{10^5\thinspace\mathrm{ly}}{10^9\thinspace\mathrm{ly}}$$
$$\theta = 10^{-5}\thinspace\mathrm{radians}$$
Now, how much can we see Pluto? It has a diameter of ~2400km, and when farthest from Earth is around 48AU in distance. So it has an angular size of
$$\theta = \frac{2.4\times 10^6\thinspace\mathrm{m}}{7.2\times 10^{12}\thinspace\mathrm{m}}$$
$$\theta = 3.3\times 10^{-7}\thinspace\mathrm{radians}$$
$$\text{linear ratio} = 30:1$$
$$\text{area (square) ratio} = 900:1$$
This means that compared to a fuzzy distant galaxy, Pluto covers about 3 orders of magnitude less angular area in the sky. No wonder that it’s more difficult to image.
Moreover, at these angular scales you run into the diffraction limit. For visible light and for an aperture the size of Hubble’s, you can’t expect to resolve features that are significantly smaller than about $2.5 \times 10^{-7}$ radians across, which is comparable to the angular size of Pluto itself. To do better from Earth orbit, you'd need to get a much larger mirror.
In fact, if you look at this image from NASA showing the Hubble extreme deep field:

at full size (976 pixels wide), then by my calculations the most zoomed in portion in the upper right is about $2.5 \times 10^{-7}\thinspace\mathrm{radians/pixel}$. In such a picture, Pluto would be just a bit bigger than 1 pixel.