Good question, actually. Was it the lack of mechs? The confusing mixture of knights and futuristic technology? Or maybe no one liked the combination of the app game gimmick? (Which flopped because people just scanned Nexo shield power images from the internet instead of buying the actual product).
Either way I wasn't busy with lego when it was released, I would've loved the five-color-element thing and especially the collectible shields. I actually bothered buying them all (even the Forbidden Power ones) separately from vendors, I just need the Merlok powers now and potentially some non-polybag shields if they exist to finish my collection.