I actually don't know. It's most probably a combination of the fact that the city is stunning but also the (soppy alert) hilarious and enduring memories of the place (we stayed at Wild Rover, it could possibly be the reason why.) We also arrived in Arequipa just before it was celebrating its birthday and preparations for the festivities were in place. It's a shame we weren't able to stay around for the celebrations but unfortunately our time travelling had almost come to an end.
Oh and did I mention the food? We went to Mercado San Camilo and the food there is AMAZING and pretty cheap as well. I got this mountain of ceviche for around 4 soles ($1.75) Incredible.