For some of the original daylily species, bees were/are the primary pollinators. For some, nocturnal moths were/are the primary pollinators and for others butterflies were/are the primary pollinators.
In the original species there typically are a number of different characteristics that have been selected/evolved to fit the natural pollinator. These are called the pollination syndrome. So a plant may open its flowers at night and have a strong scent to attract night-flying moths. Another plant may open its flowers during the day and have special colours to attract butterflies. When these two species are crossed for several generations the characteristics become mixed. For example, some of the offspring might open their flowers at night but do not have strong scent. Others might open during the day but not have the correct flower colours. The seedlings do not have appropriate pollination syndromes for successfully attracting their pollinators.
The small flowered cultivars such as Stella presumably still have enough characteristics (pollen for bees for example) and their flowers are small enough that bees can successfully pollinate them.
Larger cultivars may have lost their scents or not have strong enough scents or may open at the wrong time of the day or have the wrong flower colours, etc. fo attract their pollinators.
If you grow daylily cultivars that are not many generations removed from the species then you will find that they are more likely to be naturally pollinated than modern tetraploid cultivars, for example, Tetrina's Daughter has its flowers naturally pollinated by night-flying moths here. Ophir has a few of its flowers pollinated by night-flying moths also.
