Brown and Batygin, the authors of the paper on the possible planet, have a webpage addressing this.

A few reasons not already covered:

• It moves quite slowly - the authors estimate 0.2-0.6 arc seconds per hour - so standard surveys may not notice the movement and fail to recognize it as a solar system object.

Eris, which is the most distant confirmed object still known in the
solar system, moves at a speed of 1.5 arcseconds per hour, which is so
slow that it was missed the first time around. Most surveys of the
outer solar system would not be able to find Planet Nine, even if it
were quite bright, as they would just think it is a stationary star.

• If the planet is near aphelion, it might be an order of magnitude further away than any major or minor planet we've found so far (excluding exoplanets, which are found by methods that don't apply in this case). The authors suggest an aphelion between 500 and 1200 AU. For comparison, Pluto is at 30-50 AU, while Eris at around 100 AU wasn't discovered until 2005. The potential 9th planet would be far larger than Eris, but is also likely to be much further away, and thereby fainter.




• The WISE survey eliminated Saturn-sized planets within 10,000 AU, and Jupiter-sized planets within 26,000 AU. But the potential 9th planet is far smaller than those. WISE has also done a more sensitive search, which would pick up Neptune-sized objects, but that search has so far covered only a limited part of the sky.




• The planet will be far harder to spot if it has the Milky Way in the background - there are too many stars potentially drowning out a faint object.

Here's the authors' summary:

Estimated orbit for the putative 9th planet. The horizontal axis is the right ascension. The colored segments are regions where it should have been found by existing surveys.

Illustration by Brown and Batygin, assuming fair use applies.

The biggest unexplored territory is where, statistically, it is most
likely to be: near aphelion. Sadly, aphelion is also very close to the
Milky Way galaxy. Ugh.

So where is it? Probably distant. 500 AU+. Probably fainter than 22nd
magnitude. Very possibly in the middle of the Milky Way galaxy.

Now go find planet nine.

More details on the authors' webpage: http://www.findplanetnine.com/p/blog-page.html

Finally, the gravitational dominance of the Sun reaches halfway to the nearest star. There's still plenty of unexplored territory for planets smaller than Saturn to hide in.

Note the log axis. We have a good map for the inner 50 AU, and are starting to find objects around 100 AU, but solar system objects might exist all the way to the outer edges of the Oort cloud.

Illustration from wikipedia.