why does pluto have a high orbital inclination

I think the best answer is that there is no particular reason.

In any planetary system, most orbits tend to stay close to a common plane - close, but not exactly there. Even the big planets are slightly out-of-plane with each other, but the differences are tiny.

As you move away from the bulk of the planetary system's mass, smaller bodies tend to be more random. They are more easily deviated from encounters and near misses. Their origins might be different from the rest. If they get close to the outskirts of the system, that's a more unruly place, and orbit-changing events are more likely - especially for small bodies. Finally, the whole distribution of orbital planes has a bit of a stochastic (random) component anyway, and there will always be outliers that are quite different from the bulk.

In theory, even some larger planets could be significantly "out of plane" (much more than they are right now), but that's unlikely and it tends to be smoothed out over large periods of time.

So, Pluto is different because it's smaller and it's not really a planet, but don't attach too much significance to it.

Try to install Universe Sandbox and play with some of the included scenarios. Big systems bound by gravity can be pretty complex, and completely orderly configurations are rare.