Because Hyoyeonand Taeyeon clearly have diffrent mindset and lifestyle from each other. They are not like the closet one in SNSD but they're totally fine. Hyoyeon just said at the beginning of this year that she wanted to know more about Taeyeon because Taeyeon always kept her thoughts inside.
Recently the posts on Hyoyeon instagram are predicted to be written by Taeyeon to promote the group (Taeyeon avoiding using her instagram at the moment) because Hyoyeon rarely does things like that. They cant hate each other to death if Hyoyeon can let Taeyeon use her phone and SNS like that lol