Because no one else flies the A380 yet from SYD-BKK-LHR (i.e. Thai).
Singapore run their A380s daily from MEL/SYD to LHR via SIN, so that's exactly what Qantas will do.
Then once Thai start flying the A380 from BKK-LHR, funnily enough you'll probably see Qantas do the same!