I've explained this in another thread, but am happy to talk about it again.
From when we announce Construct 3 to when we release Construct 3 is a period of many months. During this period, a risk we identified in advance would be that everyone stops buying Construct 2 to wait for Construct 3. If this happened, we would be haemorrhaging money which would put a lot of financial stress on the business.
It's a risk we obviously wanted to avoid, so we decided to let anyone who buys Construct 2 get their first year of Construct 3 for free so that the buyer wont hesitate if they are wondering about Construct 3.
This is not at all a slight at our current customer base, we consider a lot of Construct 2 purchases now as basically pre-orders for Construct 3.