I don’t know the stats but I would guess more people have DVD players then Blu-Ray, so it makes sense for libraries to rather offer DVDs. DVDs is also one of these things that is good enough. The jump in quality between DVD and Blu-Ray is very unnoticeable (when fully immersed) compared to e.g. between VHS and DVD (or even between vinyl and CD).
I don’t know the stats but I would guess more people have DVD players then Blu-Ray, so it makes sense for libraries to rather offer DVDs. DVDs is also one of these things that is good enough. The jump in quality between DVD and Blu-Ray is very unnoticeable (when fully immersed) compared to e.g. between VHS and DVD (or even between vinyl and CD).