Many reasons. On one hand, I believe that the fundamental action of "launching" an application has to be as frictionless on the Cloud as it is on Mobile or Desktop. That's the mission of the company.
When developing products or APIs, I was really frustrated with how long everything took. Managing packages, containers, registries, images, certificates, DNS records, scaling groups, load balancers. There had to be a more adequate abstraction.
We realized that if we set up the boundaries of the HTTP protocol as our constraint, we could dramatically simplify a huge part of the paradigm. If you could imagine the Cloud as one big computer, Now is the CPU. The hard drive can be S3 and the memory Firebase, for example, for many applications.
We also identified a unique opportunity to speed up the build process. We recognized that over time even simple JavaScript applications had quite an involved process for downloading dependencies and compiling your project.
We want to move that build step to the cloud so that you get reproducible deployments very quickly.