To be fair it's not really all of a sudden, according to Wikipedia;
More recently, clubs that are knocked out of the qualifying round and (prior to 2024–25) the group stage of the Champions League can also join the UEFA Europa League, at different stages (see below). Formerly, the reigning champions qualified for the Europa League to defend their title, but
since 2015 they qualify for the Champions League. From the
2024–25 season, the winner of the Europa League can no longer defend their title as they automatically qualify for the Champions League league phase and teams cannot be transferred from that phase to the Europa League.