Interesting thread. To be honest, historically Maiden has never been a particularly riff heavy band compared to other metal acts. Of course there have been some classic riffs here and there and Steve certainly writes fewer and fewer of them. Generally there are plenty of songs, often in the choruses, where the guitars are just chugging along with the necessary power chords while Bruce sings and a guitar doubles him. Other bands tend to feature more rhythmically complex non-lead guitar playing, while Steve (nowadays) tends to go for simplified rhythms.
Another thing that plays a role, in my opinion, is the way they write and record music nowadays. The live feel leads to semi-improvised performances. Maiden used to, and most bands do it this way, to write specific riffs for specific parts. The chorus is ought to played one way, the verse has its own riff with its intricacies, the melodic interludes as well, and so on. The Evil That Men Do has a specific chorus riff. Stratego on the other hand has the chords that should be played, but the performances are semi-improvised, featuring a gallop in the second chorus that starts halfway through the first part until halfway through the second part. Almost as if the guitarist forgot to change in time and just played around a bit. I shudder to think how messy and sloppy the intricate instrumentals of songs like Rime would've been if it was written today.