The answer to this question is simply that (classical) music is not free.
We need to pay royalties to CD labels, orchestras, soloists and other content owners. Therefore we need to ask for a small subscription fee if we wish to keep our platform free from advertisements.
The reason why we do not offer a free advertisement based type of subscription is that we want to guarantee our users the best listening experience without interruptions. Advertisements can be highly damaging to classical music if an opera or symphony with gapless playback is interrupted.