I can sympathise with this point of view, but from a bluntly realistic perspective, I imagine that the majority of Construct users are infrequent, hobbyist devs who use the tool every now and then (I'd love the data on this, it's something that Scirra track).
With this in mind, they'd lose an awful lot of money if they swapped to monthly payments. It would be very easy to imagine a situation whereby the majority of users would by a handful of month subs a year, which would cost Scirra about a 3rd of their revenue from an annual sub model.
So the compromise then would be to dramatically increase the monthly sub cost; which wouldn't work for their market (entry level game dev and education).
The current model simply makes sense - it's a very low price compared to other services. There might be a middle ground for a sliding discount i.e
1 month = £18
3 month = £40
6 month = £55
1 year = £79
However I'm confident smarter minds than mine have considered and dismissed this; it's likely too complex, and as stated, the annual price is competitive enough.