I say this having defended Destiny from all number of unfair slights and complaints, and having played it for probably hundreds of hours -- Has Bungie done ANYTHING to earn them the benefit of the doubt? Even in the House of Wolves thread there was someone saying, "Don't be silly guys of course the expansion is going to include a raid!" Then like 3 hours later they announced that there was no raid. Vanilla Destiny was content-starved...."Oh but they'll turn it around with Crota's End!". Crota's End was content-starved...."Oh but they delayed House of Wolves so surely it will be packed full of stuff!" Then the raid got cancelled.
You're far better off assuming the worst than expecting these expansions to be loaded down with interesting new content.
I'm pretty sure EVERYONE got the impression that each 'Expansion pack' in the season pass would have a Raid. Bungie certainly didn't deny or assuage that. Even their 'Work in Progress' data had the HoW stuff listed as a raid.
So, we can ascertain that at some point in time, possibly during the launch period, they were planning on having a raid for HoW. There is too much evidence that points to that. Something must've happened from launch to the release of TDB, whether feedback or issues with the raid and bugs, or whatever.
IMO, it was the general feedback that made them cutback on the raid. There's definitely a negative sentiment towards Bungie and towards the Crotas End raid in terms of design compared to VoG, and maybe the new raid wasn't looking so hot. So they downsized it and made it a matchmade activity.
After all Destiny isn't like WoW-if you only have 10-20% of the playerbase doing raids, then the expansion packs are going to lose a huge amount of value to the regular joe who doesn't or can't raid. Maybe this is a test for them to see how many people do the Arena-if that turns out more popular, and if alot of people actually manage to complete it/do well/actually participate, then we might see even fewer raids and/or matchmade raids.