It was also easy to convince users in 1998 to try a new search engine at least once. It's one click, zero commitment. At the time the majority of people still weren't using the Web regularly, so you're acquiring users from a rapidly expanding base; people that had no allegiance or bias in relation to an existing search engine. Further, AltaVista & Co sucked big time. Sifting through the volumes of spam in pre-Google search engines was a real pain.

Starting a Web/mobile email client today? Nearly everyone already has an email address, and they've likely had it for a long time. They're comfortable, they have a large index of emails at their current service, and eg Gmail is a 'good enough' product for most users. Extremely different scenario vs the one Google faced.