I couldn't spot the use of the official Wikimania logo in any part of this year's event in London, from signs to booklets to social media.

As far as I know, this is the official Wikimedia foundation copyrighted logo for Wikimania: https://wikimania2014.wikimedia.org/wiki/File:Wikimania_logo_with_text.svg

Didn't see any bit of this logo totally anywhere whatsoever!

Any rationale behind?

The logo used (or I saw) throughout this year's event is this: https://commons.wikimedia.org/wiki/File:Wikimania_2014_Shard_logo_v3_with_logotype_and_date_(small).svg authored by Edward.

rexford | google.com/+Nkansahrexford| sent from smartphone