Batygin and Brown hypothesised a Neptune-mass object orbiting on a highly eccentric 10,000 - 15,000 year orbit as a solution to an observed "shepherding" of Kuiper belt object orbits (because they observed what looked like a lopsided distribution in their orientations).

As for why it hasn't been observed, if it exists, is that the region it would be in its orbit is huge, as in, dwarfing most of the solar system. That far out, from the planet's perspective the sun would be nearly indistinguishable from other stars, so the planet would be very, very cold and have next to no reflected light.

So you'd be looking for a black needle in a gigantic dark haystack who's only clues about where it is is based off of how other needles you've found have been distributed in the gigantic haystack.

When we find exoplanets, its largely because they blot out the light of their parent star, or in the case of particularly massive planets, they can make their host star wiggle.
With the hypothetical planet 9, we have no host star to helpfully observe and watch for an occultation.