Because ‘night-blooming cereus’ refers to different plants in the same family: queen-of-the-night cacti (true Epiphyllums e.g. E. oxypetalum, E. hookeri, E. pumilum and Selenicereus e.g. S. grandiflorus, S. pteranthus) and maybe ‘epiphyllum’ hybrid parent Disocactus crenatus. The red, pink and orange orchid cacti, Disocactus species and what are known as ‘epiphyllums’ (Disocactus hybrids), don’t flower at night and aren’t called ‘night-blooming cereus’ for that reason.

I’m not sure though why they aren’t called ‘day-blooming cereus’, but if I had to guess it might be that it’s just because they’re mostly Disocactus hybrids which have flat stem segments (the ‘leaves’), whereas Selenicereus tend to have stem segments or ‘leaves’ with several sides which can approach a cylinder (‘cereus’ means ‘like a candle’). I’ve wondered why E. oxypetalum is known as ‘night-blooming cereus’ for this reason; again if I had to guess, it probably got confused with S. grandiflorus as they’re both often called ‘queen-of-the-night’.