If they released them on DVD, they wouldn't sell as many Blu-rays. Simple as that.
By offering the series only on Blu-ray, they've got at least some people going "Well, guess it's time to upgrade!" Those people would be content to just stick with their old DVD players if they offered it on both formats.
By offering the series only on Blu-ray, they've got at least some people going "Well, guess it's time to upgrade!" Those people would be content to just stick with their old DVD players if they offered it on both formats.