As others wrote, using again x86 architecture, BC is a lot easier to achieve. Using it the first time this gen gave a mix of pros and cons, easier to implement overall architecture, easy ports to and from PC, wider range of engines and dev tools, with probably the only con of larger die size compared to a RISC CPU or APU with the same performances using the same production process, using again x86 adds the aforementioned pro almost automatically, even without patents about it.
Choosing again x86 is a natural coice, as PS4 and XBOne used lightweight Jaguar CPU cores, so Zen2, or even better Zen3, will be able to offer a performance boost big enough for a new gen even in their lightweight implementations, while unless something changes, scaling up ARM architecture above performances and consumption mobile market typically requires would be probably more expensive, maybe it will become the viable choice if and when PC market will drop x86 and adopt it, it could happen later, but it's very unlikely it do it by the time MS and Sony start their next gen.