Why didn't Indonesians/Indian Ocean traders bring marsupials to Asia and placental mammals to Australia/New Guinea?

Because it wouldn't have done either of them much good.

Australia didn't house a settled agricultural society, with major population centers to trade with. Where it was closest to Indonesia, it was instead until quite recently very sparsely populated with roving bands of hunter-gatherers. According to McEvedy and Jones, there were in the neighborhood of 250,000 locals spread over the entire continent.

The land itself wouldn't have been great for Indonesians to colonize either. Their traditional crop package doesn't grow well in Australia due to various combinations of climate and soil issues. The Indonesians of course were no dummies, and would have happily colonized the useful areas of Australia within their reach, as they did the rest of Indonesia, were it feasible to do so.

Notice from the climate map above that only small northern bits of Australia had climate similar to the rest of Indonesia. Those areas however apparently have soil issues that make farming rice in them commercially difficult even today.

Although attempts to grow rice in the well-watered north of Australia have been made for many years, they have consistently failed because of inherent iron and manganese toxicities in the soils and destruction by pests.

It is also tempting to speculate that if such an effort at crop domestication could have worked, someone among the native Australians would have worked it out in the 50,000 years they have lived there. They aren't dummies either. The best they could work out was fish farms in the far southeastern (opposite) end of the continent.

The native fauna would not be particularly useful to an ancient Indonesian to cart off as well. They had their own domesticated animals that suited their purposes just fine. Even today with modern technology and population levels, no native Australian mammal species has been domesticated in any commercially important way. There is a brisk trade in kangaroo meat today, but its largely from slaughtered wild kangaroos.

There are actually a number of placental mammals which can be found across the Wallace Line in Australia and New Guinea. The one which probably is most relevant to the question about deliberate human involvement in the process would be the dog (or dingo), which was most likely introduced and dispersed into New Guinea and Australia from South-East Asia by humans who found them useful in various ways.

The Dingo

The dingo (Canis familiaris, Canis familiaris dingo, or Canis lupus dingo) is a dog (Canidae) that is found in Australia. Its taxonomic classification is debated. It is a medium-sized canine that possesses a lean, hardy body adapted for speed, agility, and stamina. The dingo's three main coat colours are: light ginger or tan, black and tan, or creamy white. The skull is wedge-shaped and appears large in proportion to the body.

The earliest known dingo fossil, found in Western Australia, dates to 3,450 years ago, which led to the presumption that dingoes came to Australia with seafarers prior to that time, possibly from south-west Sulawesi in modern-day Indonesia. Dingo morphology has not changed over the past 3,500 years: this suggests that no artificial selection has been applied over this period.

The dingo is closely related to the New Guinea singing dog: their lineage split early from the lineage that led to today's domestic dogs, and can be traced back through the Malay Archipelago to Asia. A recent genetic study shows that the lineage of those dingoes found today in the northwestern part of the Australian continent split from the lineage of the New Guinea singing dog and southeastern dingo 6,300 BC, followed by a split between the New Guinea singing dog lineage from the southeastern dingo lineage 5,800 BC. The study proposes that two dingo migrations occurred when sea levels were lower and Australia and New Guinea formed one landmass named Sahul that existed until 6,500–8,000 years ago.

From Wikipedia

Rodents

Wikipedia also contains a list of rodents which have made their way to Australia over the last few million years, although probably not as a result of deliberate human action (as the question seems to be specifically addressing).