If I understand the OP right, this was not an acute case. And AY themselves say: "In the case of your flight AY55, the cancellation was caused due to lack of aircrafts as no spare aircrafts were available. This was due to mouse trapping operation on one of our aircrafts previous flight."

This means the cxl was due to commercial reasons, ie they choose which flight to cancel in order to minimize their losses. In this case, they are obliged to pay the EU compensation. They won't, however, because there's no effective way to force them to.