I learned in high school lit that sentence length is an artistic choice as meaningful as word selection: long sentences can reflect stream of consciousness, recursive thought, associative or digressive exploration. Short sentences can reflect anxiety, urgency, vigilance, cognitive compression.
There are a lot of factors that have led to the decay of long sentences. Scientific writing norms, ubiquitous style guides like Strunk & White, modern distraction/multitasking/short(er)-form content, and my favorite, impoverished education - and the concomitant lack of trust in the reader on the part of the author.