There are three possible reasons that I see.
1) What YYZ717 said, it is very true that the route is heavy in the summer and light in the winter, although they were doing pretty well witht eh once weekly service last year.
2) They are planning to introduce a simmilar service to the one that they have to the US, whereby they codeshare on an Uzbekistan airways 767 from Kyiv to Toronto selling half the seats as Air Ukraine, and the other half as Uzbekistan airways to and from Taskent to Toronto via Kyiv. Uzbekistan has recently applied for Canadian flights, and these two announcements seem to be close together.
3) The three major airlines in Ukraine, Air Ukraine (the once that comes to YYZ), Ukraine International (All 737 fleet domestic and european flights, and Aerosvit (European Middle East all 7373ops). Are in the process of being merged into one national airline, the North American route cuts may be a way of consolodating their assets/losses, and regrouping before starting new services to Canada, most likely using new equipment(767).
What ever the reason, I hope the come back soon..
"Clive Beddoe says he favours competition, but his actions do not support that idea." Robert Milton - CEO Air Canada