Lack of privacy:
Because the fireplace is in the common room. There is complete lack of privacy.
Harry heard a tiny pop! in the fire behind him and knew Sirius had gone. He watched the bottom of the spiral staircase. Who had decided to go for a stroll at one o'clock in the morning, and stopped Sirius from telling him how to get past a dragon?
It was Ron. Dressed in his maroon paisley pajamas, Ron stopped dead facing Harry across the room, and looked around.
Harry Potter and The Goblet of Fire, Chapter 19, The Hungarian Horntail
Easy to Eavesdrop:
People can intercept the discussions taking place over fireplace
Hermione gave a horrified gasp and leapt to her feet, still staring at the fire.
A hand had appeared amongst the flames, groping as though to catch hold of something; a stubby, short-fingered hand covered in ugly old-fashioned rings.
The three of them ran for it. At the door of the boys' dormitory Harry looked back. Umbridge's hand was still making snatching movements amongst the flames, as though she knew exactly where Sirius's hair had been moments before and was determined to seize it.
Harry Potter and the Order of the Phoenix, Chapter 17, Educational Decree Number Twenty-four
Lack of Fireplaces:
All the students in a single house share the common room. Imagine the queue near the fireplace if it is used for communications.
“Welcome to Hogwarts,” said Professor McGonagall. “The start-of-term banquet will begin shortly, but before you take your seats in the Great Hall, you will be sorted into your houses. The Sorting is a very important ceremony because, while you are here, your house will be something like your family within Hogwarts. You will have classes with the rest of your house, sleep in your house dormitory, and spend free time in your house common room.”
Harry Potter and the Sorcerer's Stone, Chapter 7, The Sorting Hat