There is not a doubt in my mind that Bruckner's music is an acquired taste. It took me almost 3 years to like Bruckner (from the hearing the 4th for the first time to finally "converted" by Jochum's 5th with RCO). Why do so many people find his music so difficult at the beginning? here are my thoughts:
1) Length His symphonies are on average very long. If you are not a patient listener (strongly correlated to how long you have been listening to classical music as a whole) you'll likely to find his music "boring", and probably stop listen to it after a few minutes into the work. But, what you are missing is his meticulous construction of his symphonies. Bruckner doesn't rely on famous melodies (not saying there aren't famous melodies in his works). He constructs his music base on motives. A strong theme + Lyrical theme + chorale theme. He repeats, and recycles these motivic material throughout his symphonies. So, if you are a poor listener, you probably will not hear the traces of these motives in the recapitulation.
2) He makes you wait for it! Beethoven's music, very fiery, and romantic. Almost instantaneously satisfies you. Dvorak's music, with its wonderful melodies, lush harmonies... there is no way for anyone to NOT whistle some melodies after hearing his music. Bruckner, on the other hand, make you wait. His cadences, his codas are probably the greatest in all of symphonic repertoire. He slowly builds his music, through dark and sinister territories and finally to light. This is exemplified in the final movement of his 9th symphony (reference to the SMCP completion, which were used by Wildner and Eichhorn), the entire last movement were full of these dark, sinister little motives. It is sometimes very unpleasant to listen to, for instance, the dissonances. But, after all that "suffering", in the coda, he rewards you with the "light" you've been seeking all along. This is in many ways, the most profound music making in the history of Western music.
3) Differences in Qualities of The Scores When you talk about Beethoven's 5th, there is only ONE. Regardless of which conductor/orchestra you listen to, regardless of their own artistic interpretations, they are still using the same exact notes that Beethoven wrote. But for Bruckner, most of his symphonies had several versions. Some was edited by himself, which improved the music overall(think about 4th and 8th); some however, weren't. The ignorant fools who "revised" his works should burn in hell for that! (yes, i am talking to you, Ferdinand Lowe) Herr. Lowe's crowning achievements in messing up Bruckner's music is demonstrated in Hans Knuppertbusch's recording of the 9th symphony. Where the brilliant mysterious/unconventional pizzicati in the scherzo was doubled up by woodwinds instead.... This is just ONE examples of such appalling acts of vandalism. So, naturally, conductors are given the task to not only playing the music, but to actually decide on which version they use. Sometimes, this can lead to great confusion, and difference in the quality of the performances.
4) A Lack of Interest You must all realize that, there hasn't always been a wealth of recordings available like today. Today, we are kind of experiencing a "Bruckner Revival", more and more people are beginning to explore his music. But, in the old days, his music was largely ignored. The reasons behind this is complex; a)it had something to do with the difficulties in performing Bruckner's works. Orchestras in the old times were not nearly as sophisticated as the orchestras we have today. And, Bruckner's music can be a pain to play. b) When the Nazis were gaining power in Germany, Hitler's propaganda man Goebbels introduced to Hitler Bruckner's music, and Hitler, with his perverted logic, included Bruckner's music in his mass propaganda. So, naturally, after WWII, there is this very bad "guilty by association" going on, people assumed fallaciously, that Bruckner's music is "Nazi" music.
5) Conductors and Orchestras No where is this more evident, in Bruckner's recordings. Because of the sheer complexities of his works, Better conductor/Orchestra= Better music. A symphony #3 by Joe Nobody can sound dull and boring. A symphony #3 by Georg Tintner can sound absolutely astounding.
6) Gaining Access This is never that BIG of a problem, since today's technologies really permitted us listeners to things that is totally unimaginable 20 years ago. Nevertheless, I should say that if someone is incapable of finding Bruckner's music, he/she should always pay a visit to his/her local library(how i got started). Or, go to Amazon, and simply typing up "Bruckner", and a wealth of recordings will appear... Even for people, who'd rather not spend any money on Bruckner's music, there are still many opportunities to hear his music. There are websites out there that can let you hear Bruckner's symphonies for FREE (legally), albeit for a limited quantity or time. Sites, such as Rhapsody.com or Naxosmusiclibrary.com have extensive Bruckner Collections, and it doesn't take more than a couple of minutes to access them, for no charge whatsoever.
There you go, my explanation and some suggestion to the struggling music listeners out there. I only have one thing to say now, the ability to listen and appreciate slow music, (in my experience) strongly correlate with my own levels of maturity. So, be patient, and you'll find Joy in Bruckner.