For what it's worth, I feel most of the output from these studios is still generally okay/good. And not like...fundamentally fucking broken and awful. Although Fallout 76 is apparently a real contender for those titles, and I don't intend to find out first hand.
That being said Fallout 4, Dragon Age: Inquisition, Mass Effect: Andromeda, and maybe to a lesser extent Anthem (again, need to try for myself), are all average games at worst. Some loathe them, but they're not fundamentally busted, generally have a ton of content and decently impressive ambition. All just fail to come together as whole projects.
This is probably kind of a broad answer, but I think a major roadblock both Bethesda and BioWare hit were other studios essentially catching up with modern technology and the scope of the kind of games both are famed for, alongside they themselves failing to modernise and impress with new technology and concepts.
Irrespective to how you feel about either last gen (there's a real argument both made steps towards greater accessibility of their franchises versus older work, which can be seen as regression), all produced generally well rounded, original titles in the landscape of gaming. If just for the scope of their productions and the content they offered. You knew you were playing a Bethesda game. And you knew you were playing a BioWare game. And both were tailored to specific tastes.
Maybe, in this day and age, that just isn't enough any more. Other devs mightn't be making the exact same games, but there's contenders out there, and much of what both companies were know for has arguably become homogenised. BioWare isn't the only studio producing titles with immensely rich world building and lore, and absurd density of voice work. Bethesda isn't alone now producing worlds that can be explored, begging for immersion. We mightn't be drowning in exactly these kinds of games, but with Guerilla and Horizon, and CDPR and The Witcher 3, we're seeing more developers put in seriously impressive work into these massive, open, semi-RPG type games that encourage exploration and discovery. And developers are pushing harder on narrative across the entire medium, leading to more dialogue, larger casts, and more professional performances.
So with the standards raised last gen, and now others catching or outright caught up, there's a real argument that what Bethesda and BioWare are offering aren't bad, but also lack an original hook that capturing modern audiences in the way they did last generation. Maybe their games are, to put it bluntly, far too familiar and unambitious, and maybe the market has moved on to other titles. Their games are instead at best technologically competent and updated clones of their design template from the previous generation, with most attempts at ambition unfortunately squandered under either technical barriers or production difficulties, which again draws comparison as other developers simply do better with the same ideas. Fallout 76's open world co-op RPG stuff really isn't original; the online faux-MMO survival sim genre is older, littered with titles, and developers have learned lessons the hard way Bethesda failed to. BioWare started exploring more open, quest riddled, crafting laden concepts in Andromeda and Inquisition, but again are amateurish in their expression and design versus more experienced developers.
So yeah, I dunno. I do think both are home to very talented developers. And I don't think any have output I'd label catastrophic (I mean, again, Fallout 76 aside). But I do think both may legitimately be victims of time, and a failure to adapt to growth.