Because there is a lot more of dated dependencies, probably something puts a constraint on postgrex that makes it unable to update.
If you put {:postgrex, "~> 0.13"} in your mix.exs and then mix do deps.unlock postgrex, deps.update postgrex again, it will tell you where constraints are comming from.
PS: Also do not put >= as a version constraint unless you really know what you are doing.
PPS: ecto 2.0.6 had a constraint on postgrex that basically narrows it to 0.12.x.
PPPS: You really should ditch that old, outdated and deprecated guide in favor of the new hexdocs hosted: Up and Running — Phoenix v1.8.8, how have you actually found that other one? Those aren’t linked from the phoenix homepage anymore for years…
