What could be potential reasons for a) the object only now being discovered (200km in size expected) b) it spinning opposite ?
The combination of size and distance is the reason it wasn't discovered earlier.
This XKCD graphic shows this pretty well:
Anything below and to the right of the areas "stuff we can see through telescopes" and "planets ruled out by the WISE survey" is too small to see from Earth. Improvements in telescopes and data processing are gradually moving those boundaries to the right.
To find a planet, you have to take photos of the sky at intervals and compare them. The stars will be in the same position in both photos, the planets will have moved. Doing such a survey of the whole sky is difficult. Doing it manually is a monumental task (it took 4 years to discover Pluto), these days computers can help with that.