Agree with every point u made. I should add something.
#5. Good looking second and third lead casts. The other 2 couples can be main leads in their own lakorn as well. The other supporting casts are attractive too eg: Por Duer, Petracha, Gosa Lek, Gosa Ban, Joi, Luang Siyuth, Clara..
#6. Every character have their own charms and strong presence in the story. It is like they are having their own lives instead of just being there in the background.
#7. Strong acting by everyone. Bella since the first ep made viewers want to go to Ayuthaya too. The rest suit their roles too.
#8. The novel itself has a good storyline I understand but we got a wonderful script writer. That's why every words each character said become talk of the town.
#9. Little touch of comedy from every character. Even annoying Mae Prik turn out to be funny and adorable with her dotting on Kade and sulking with Khun Ying Jumpa. Por Date's dad was funny sometimes too especially his confusing expression. No one imagine Janward character to be able to bring in comedy. But she successfully made many people laugh when she pretend to be sick and got up so fast when she heard Khun Rueng ma jao ka.
There are more factors I believe.