I can't speak to why transportation was chosen over other punishments for these prisoners, but I can talk a little about why Western Australia was chosen.

The Swan River colony (now Perth) was established in 1829, and initially it was established like the South Australian colony a few years later: no convicts. The goal was to create a free settlement, and avoid the negative stigma associated with a mixed community of free settlers, convicts and ex-convicts.

While there was initial excitement, when the settlers did arrive they discovered the land was not as suitable for farming as was advertised - reports sent to Britain in 1830 said that colonists were starving. This put off potential future colonists, for obvious reasons. Over a thousand settlers left in the next 5 years, and the colony fell into an economic depression as the farming industries could not grow as expected. There was also a lack of infrastructure and resources, especially labour.

The request to the colonial government for convicts to be brought to the settlement was made by landowners in 1847. Britain agreed to send convicts to prevent the settlement dissolving, and the colony became a penal settlement in 1849. Keeping a British settlement on the west coast of the continent was important to maintaining the Empire's "claim" to the entire landmass - the western part of Australia had only been formerly claimed for the British Empire in 1827, and there were fears the French could move in and claim some of it. Note that this is all based on ignoring Indigenous rights to the land, obviously.

Convicts were transported there until 1868 - over 9000 in that time period. All the convicts were men, unlike in Sydney, and many were part way through their sentences. They were primarily used for labour on government projects, building roads, bridges and public buildings including part of the Fremantle prison.

The last group of convicts to be transported to Western Australia were a different sort - they were Irish political prisoners who'd been involved in the fight for home rule in Ireland. Perhaps the answer of why transportation was chosen for them will be different to the general convict population.

Hopefully this is helpful!