Why did Toyota add the Lexus and Scion product lines to its product portfolio as separate umbrella brands instead of retaining an all-Toyota product line portfolio?

Why did Toyota add the Lexus and Scion product lines to its product portfolio as separate umbrella brands instead of retaining an all-Toyota product line portfolio?

Why did Toyota add the Lexus and Scion product lines to its product portfolio as separate umbrella brands instead of retaining an all-Toyota product line portfolio?