Inspirations: Best of many Worlds
From the vast and rich choice of technologies out there, many good things are cherrypicked into this framework. The learnings from the past decades of web development are distilled into this framework, which itself stands on the shoulders of giants. The major inspiration sources are:
Javascript: the most popular programming language in the web development space. Everyone knows it more or less and you get access to the biggest collection of libraries of any language out there: NPM. Since JS runs on both, clients and servers (also called "isomorphic" or "universal"), this is the only language you need.
Ruby on Rails: The very core paradigm that made Rails become the striking success was Convention over Configuration. Ultra-flexible configurations were (and a decade later: are again) exhausting productivity killers and responsible for very high cognitive load for developers. CoC solves this elegantly by dictating a few rules how stuff must be organized and freeing up developers to have more time for actual development. CoC happily trades in some flexibility for big gains in productivity.
Go: The most important factor that drives Go's success is probably the tooling around the language. It may sound irrelevant but developers often spend much time every day to just format their code with tiny interactions, like comma placements or indentations (tabs vs spaces, ...). Go just stops all of this with gofmt. Seagull has it's own powerful fmt command, too.
Serverless: From servers to docker containers, to compose, to swarm and kubernetes, the complexity of "just keep my app running" has increased dramatically over the last years. The serverless architecture directly eliminates the very idea of servers, erasing the many thought chains of orchestration and scalability issues. Developers can again focus more on actually developing stuff instead of doing DevOps works all day long.
PHP: Yes, PHP. The one thing that made PHP appear so great for new developers back in the day was actually just one major thing: the deployment story. Get a "web host" for 2$ per month, FTP your "program" onto a server folder, open your web app in your browser. Ultra short feedback loop and instant gratification, nearly no costs. Exactly what new developers need to feel great right away and stick to something. Seagull leverages the FaaS paradigm on serverless, leading to even simpler and cheaper deployments than during the old PHP days.