Probably this is because Vivaldi antennas have a broad beamwidth, which allows them to receive signals from different angles and polarizations.
Furthermore, Vivaldi antennas are typically used in applications where the polarization of the signal is unknown or varies, such as in broadband communications, radar, or sensing applications. In these cases, cross-polarization performance is less critical because the antenna is designed to receive a wide range of signals with varying polarizations.