This older answer discusses why the route was used, https://www.reddit.com/r/AskHistorians/comments/41sdh5/im_a_convict_on_my_way_to_australia_in_1800_what/ It wasn't just ships to Australia, the first part was also used by East Indiamen.

Very briefly: sailing ships required reliable winds and navigation. To give the answer some perspective; Reliable chronometers weren't routinely available until about 1825, Ocean going steam ships required coal and refueling stations and on long voyages weren't really competitive with sail until the 1870s (and for long voyages with bulk freight like wheat, sail was economically competitive on the Australia - London route until 1930), and the much shorter route via the Suez Canal didn't open until 1869 and was for steam ships.