SaaS doesn't sell the way most things SEO is built around. There's no single purchase moment, no one-time transaction, no straightforward "search, click, buy." There's a free trial, a demo request, a multi-stakeholder evaluation, a comparison against three competitors, and a sales cycle that might run weeks or months before anyone signs anything.
Generic SEO treats every site like an e-commerce store or a local business: rank for keywords, get traffic, hope it converts. SaaS needs something more specific: content and technical decisions built around signups, demos, and the actual evaluation process software buyers go through, with MRR as the real scoreboard, not traffic.