I can only speculate — Sketch should be quite healthy financially, but Adobe has about $5b in annual revenue, which affords them an almost indefinite amount of staying power, especially for a market segment right in their yard.
Add to that their existing IP — they don’t need to discover anything substantially new about graphics nor about building cross-platform software that performs well. And these are not easy things. They can also leverage Adobe’s well-oiled software delivery mechanism (the people to run the CI servers alone can cost a million dollars a year), marketing and customer acquisition.
We generally don’t think about software as something that has similar economies of scale as that of real-world things like manufacturing. But it is very much there. As a quick example, the presence of Creative Cloud makes it possible for Adobe to roll out new products as they please and have them hook straight into its SaaS mechanism. Product teams can simply focus on their product and no longer need to worry about creating a billing engine, making an admin dashboard to monitor metrics, or even care about the email infrastructure, not to mention the many hundred other things that goes into running a software business.
If all of this isn’t a David vs Goliath, then I don’t know what is 😀