Put simply: cost. Collectively it's a few hundred Gb per month of traffic which is completely covered by my Linode plan.
These days I've put most of the sites behind Cloudflare, which has the effect that a lot of my media is being mirrored in their Sydney DC, which is nice because most of the readership is Australian -- the same reason I went to the Tokyo centre this time.
The Tokyo DC isn't a universal win for Australia. The results are actually pretty bimodal depending on your ISP.
From Perth, iiNet routes directly to Asia, and it's a big win. But Amnet routes AU -> US -> Asia and it's a big loss.
It's a big enough issue Australia-wide that Blizzard sells a dual-region Starcraft 2 in Australia, because (ping to battle.net Singapore for some) > (ping to battle.net US) > (ping to battle.net Singapore for others) to a significant degree.
These days I've put most of the sites behind Cloudflare, which has the effect that a lot of my media is being mirrored in their Sydney DC, which is nice because most of the readership is Australian -- the same reason I went to the Tokyo centre this time.