I doubt there is only one or even main reason for this. It is more a decision based in several distinct factors, that Sony thought could be dealt or exploited by NEO.
They definitely considered the possibility of a stronger nintendo machine. Wii U was failling badly, but Nintendo was not giving any signs that they were over with the console market. If they were, SONY would be the first to know, being one of the most important places for a third-party Nintendo to go.
That said they also considered, to name a few:
- Possibility of Microsoft trying to react to the current situation with a new machine
- Possibility of new competitors: SteamMachines, Apple, Amazon
- Necessity of more power for VR
- Consumer habits: people are willing to buy new smartphones every year, and they are just as expensive as consoles. While we are talking about two very different markets, this consumerist mentality has already proved to be present in many tech-related market. It is a great opportunity to sell
Besides all that, this strategy was most likely saw as an option since the beginning. Instead of trying to sell a $600 stronger machine from the go (like the PS3), first capture the userbase with a marginally better machine for a competitive price and than release a really powerful version, but more expensive.
This does refers, at least in some level, to the Apple/Samsung strategy of releasing two versions of their new smartphones, but in this case it has less to do with userbase.
