Why do traditional astrologers disregard the 3 outer planets?

Good question.  The short answer is "Most don't."  I heard Robert Hand describe himself as "A medieval astrologer who uses Hellenistic when it doesn't conflict with the medieval, and I use the outer planets."

The outer three do not fit into the dignity scheme and it is the dignity scheme that differentiates traditional from modern.  Take Mars for example.  In the tradition, Mars has two signs that it rules, Aries (masculine, hot and dry), and Scorpio (feminine, cold and moist). If we give Scorpio to Pluto, Mars loses something it shouldn't and the astrologer loses something about Mars, and Pluto never has something Mars does. In fact if you look at the rulerships assigned to the outers, they weer all assigned to something else either specifically or by extension.  Computers are assigned to Uranus, yet they are, boiled to their essentials, communication and calculation devices, and that's Mercury.  With the right mindset, the outers are not needed.  

Traditional astrologers will take note of the outer planets.  Some use them like they use the fixed stars. Some just use them as moderns do, but without sign rulerships or other dignities. Most mundane astrologers, traditional and modern will use the outer three and note when they are prominent, e.g. on an angle in the Aries Ingress.

Finally, since natal astrology is about all anyone cares about, the outer three stay in signs so long that tens and perhaps hundreds of millions of people have the same planet-sign combination with the outers.  When transiting Saturn squares Neptune, it does so for half a billion people or so.  They can't all have the same experience. This is especially true when the others aspect each other. There is nothing "personal" about them.  

But mostly it's the rulerships and dignities that represent the problem.  In order to fully grasp the problem, one needs to understand traditional astrology and the worldview that gave it life.  That worldview no longer exists and moderns that make no effort to understand that, will never get why the outers don't dominate traditional astrology.

Finally there is this: why do moderns emphasize the outer three in the extreme?  Pick up a copy of "The Mountain Astrologer."  Each and every issue is a long feature of one of the outer three.  It's as though Mercury no longer exists, and yet everything they write is about the almighty personality. They could use a good dose of traditional. 

WESTERN astrologers do not disregard the 3 outer planets.

The Signs of these planets don't mean much, but aspects they make to the personal planets can be highly significant.

And, if angular in the chart, very powerful.

And yes, for transits and synastry, the outer planets can yield very important information.

In over 40 years of attending astrological conferences in North America, I have never run across a lecturer who did not include the outer planets.