[dietrichepp]

Wordpress does not abstract the database, so migration to SQLite would break tons of plugins.

Wordpress is also a PHP application, and it's common to have multiple PHP processes running in parallel, which makes it much harder to use SQLite.

[spilk]

Wordpress does attempt some abstraction of the database via its various wrapper classes/functions like wpdb, WP_Query, etc. It's definitely not ORM-level abstraction by any stretch of the imagination, though.