Like Mako said, it was probably just preference, but it was also a good choice IMO. A lot of web platforms are programmed in interpreted languages like PHP, Ruby and Javascript (on the server side, via Node.js). Rust is a compiled systems programming language. As a result, the Lemmy back-end runs as native machine code, with no need for a virtual machine or interpreter. It runs faster, and it scales better.

This makes development more complicated, but I think the pay-off is worth it. It is also worth noting that Rust was originally developed by Mozilla for use in the Firefox browser, and had a vibrant selection of libraries for web development from the very early days. It is particularly suited for web-facing applications thanks to unique features like the "borrow checker," which prohibits entire categories of memory access and concurrency errors which are typically exploited by malicious inputs. It still doesn't prevent you from shooting your own foot off though.

Based on anecdotal experience, Lemmy is at least an order of magnitude more efficient than Mastodon, which is programmed in Ruby.