Installing a seat in an active aircraft is not easy, you need full certification for that configuration. More on STC is here: https://www.easa.europa.eu/en/docume...e-certificates

After the 2010 Koito seat certifcation scandal, these permits are no longer handed out easily. So just quickly installing something in an aircraft and sending it in active service is nearly impossible. The inflight test of the B/E Diamond for C and B/E Oasis for F in the end had very little value for LH as it was a massive effort for very litte data/feedback. Installing the seat in the mockup at FRA is much cheaper. They did that for the new Allegris C in FRA in a dummy B779 some time in 2017.