Mostly because player makers don't keep updating after a year or two. I have a very old Roku and it gets updates which is very important when say Google changed how YouTube worked. Players just removed YouTube apps and said it was Google's fault.
I joined my local PBS station as a sustaining member to access their on demand contant - PBS PASSPORT, I believe. It turns out that the ONLY smart TV they support is Sony. So they told me to buy a standalone device such as ROKU. However - & here comes my question - my research seemed to indicate that most internet ready blu ray players work better than devices such as Roku???? So Can I, through my Sony Blu Ray player, access the PBS.ORG app to allow me to watch the PBS content on my Visio TV to which my blu ray is attached? Apparently my Visio uses "Opera" & Sony "Android" & Android is supported by PNS ??? whew, confusing 
Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic