Why is it difficult/how to make it easier for Americans to understand Australian accents?

I occasionally talk to people from various parts of Australia. I find that there are different thicknesses in Australian accent. The thicker the accent, or the faster they talk, the harder time I may have understanding them.

This applies to folks from the UK too. Different accents or speaking speeds can make it hard for me to understand.

This also applies to Americans. Thick enough accent (Boston, New York, deep South for example) and I'll likely have to ask you to repeat yourself.

Non-American English speakers generally have an advantage over Americans when it comes to understanding the others accent: They are usually exposed to American accented English due to the proliferation of American cinema. The opposite does not happen nearly as much.

So to answer the first part of the question directly (why): There is a general difficulty for English speakers to understand other English speakers when an accent is thick to the listener's ears.

How to make it easier to understand Australian [or any] accent: Watch American movies and talk like them! Just kidding. Talking more deliberately (slowly, but not in a way that insults) and avoiding slang/colloquialisms will help.

Understanding unfamiliar accents can be very difficult. Most Americans have never talked to someone with an Australian accent.

Personally, I've gotten a lot better at understanding Australian accents due to some Australian friends and a few trips there, but the first time I heard two Australians talking to each other (I was 21 and traveling in Europe) I could barely understand one word a sentence.

It has primarily to do with exposure to different dialects of English. The number of Americans who have traveled abroad is but a small percentage of the population vs Australians, New Zealanders and British. The USA is so large and diversified, that many American travelers have a multitude of things to see and do during their holidays in country without the need to jump on a 7 hour plus flight. And while many travelers come to the USA, the chance of an average American encountering foreigners is rather limited unless they work in tourism. And even those workers may only encounter the occasional Australian or Englander or any other specific nationality.

And as was previously mentioned the massive numbers of Hollywood films being shown around the world does give the rest of the globe a taste of Americanisms giving them a bit of an understanding advantage.

I edited my answer to concentrate on the OP's original question, as there seems to be too much argument on who has an accent vs non-accent aspect.

As a native-born American English speaker, I find that I have to 'tune' my hearing if I haven't heard a particular accent before or not heard it for a while. Let's just say that the first time I saw an episode of Taggart (Glaswegian accents), I understood almost nothing. Even after watching multiple seasons, there are still some words that I can't make out because they're slang, even though I can understand the accent.

I can now usually identify an Australian speaker, but still have trouble differentiating some Kiwi / New Zealand and South African accents, and that's after living with a South African partner for 8 years.

I'm English, and had similar difficulties when I first moved to California. It is important to remember that formal written English is much more international than spoken English.

If you are having trouble being understood, try to move your spoken English towards written English. Enunciate each word, which will slow down your speech. Avoid slang.

Australians with a broad accent tend to omit letters from words when speaking. For example, mispronouncing "Australia" as "Ostraya", which ends up sounding like "Austria". Additionally, many Australians will merge words together, failing to clearly delineate between the end of one word and the beginning of the next, and make excessive use of colloquialisms (slang). For someone unfamiliar with Australian English, this just sounds like mumbling.

Contrast that with Americans who will often speak very clearly, especially when speaking with someone who isn't from their own community or is simply unfamiliar, and it's a recipe for misunderstanding.

I'm from Melbourne, and I had no problem being understood in the states, but then I completely omitted slang and spoke "properly" except with people who knew me well.