My understanding (from the outside, so probably wrong) was that a while back they had an expired certificate that broke all add-ons. This caused the existing Firefox for Android (Fennec) to get a bunch of 1-star reviews on Google Play Store, and _that_ (plus the complaints elsewhere) convinced them that not having add-ons in Firefox Preview (Fenix) was a bad idea.

Corrections welcome — I've probably missed something somewhere.