PE ratios are a measure of earnings risk and potential future earnings.
The technology sector, as RB pointed out earlier this weekend, makes up a far more significant portion of the US market than the ASX. The technology sector has higher perceived earnings potential than what banks, supermarkets and property sectors do. The ASX is heavily weighted to those lower PE sectors. If we had more CSLs or REA Groups then our index would look a lot different. These are high PE businesses and they make up a small part of our index. With all the headwinds our banks and supermarkets are facing (through government policy and competition) it may well be the case in future generations.
It helps when the US government holds a majority in both houses and is pro Wall Street. This is far from the case in Australia.
Note that American banks trade on very similar PEs to the ASX banks, as do their supermarkets and industrials. All sectors in fact, just a different mix and sentiment.
(20min delay)