Why is the petrarchan sonnet called by that name?

Spenser is usually called The Poets' Poet. This is mainly because Spenser uses a very complex rimescheme (the Spenserian Stanza) and delivers a very leisurely poem (The Faerie Queene) which takes its time to present an ornate and highly decorative storyline with complex allegories and symbolism, and many figures of speech.

Critics assume that 'ordinary' readers are usually more interested in character, plotline, and locale: but all of these are secondary considerations in The Faerie Queene, where the beauty of the versification and the language are clearly what Spenser is most interested in.

So the assumption is that The Faerie Queene will appeal mainly to readers who are more interested in language than in what is being said. Which makes Spenser The Poets' Poet.