Why are red roses the most popular?

My thoughts are, from years of taking orders is, that is what people have heard of are familiar with. Kinda like us southern folks asking for a Coke, but meaning just about any brand of soda.
Red roses are safe and usually readily available and most guys are familiar with that so there it is.
Of course flowers for Valentine's Day are mostly purchased by men for women. It is easy to see why so many are sold when you think about it. That even carries over to the term single rose or dozen roses. Refer back to the Coke comment earlier.

When people become more educated and familiar with flowers in general then they become more adventurous with the floral order. Much like going to a new restaurant, folks usually try the tried and true until they become familiar with how different items taste and so on.

Flowers are beautiful and meant to be shared. They bring a lot of beauty and happiness as gifts.
So don't forget your sweetie, whether it is with a single lovely red rose or a stunning display of a dozen, it really is hard to mess up.