Just to offer some context in layman's terms:

1. These objects are very, very far from the sun (and Earth).

2. These objects are planet(oids), not stars, so don't produce their own light.

3. They are probably very cold and don't emit much in the IR spectrum.

4. They may have low albedo, coupled with their distance from the sun, means they reflect very little visible light.

5. Space is huge. Trying to spot a tiny, dark, object many AU from Earth is no easy task if you don't know exactly where to look.