The monthly subscription is an entirely new and more accessible plan for Construct 3, representing a much lower cost to get started. As with pretty much all subscription services, you get a discount for going with the annual option. I mean, we could have hiked the annual subscription price up a lot to make monthly 1/12th that value, but I don't think that would have been much more popular! 😉
If you only use Construct for a couple of months, you save a lot of money. If you plan to use it long-term, you can pay for a year and save a lot in comparison to paying monthly. I don't see how this is a problem - how long do you plan on using Construct for?