Why is Senketsu the only Kamui that can talk? There are three Kamui in the show: Senketsu, Junketsu and Shinra-Kōketsu (Omnisilk Kōketsu). 





Shinra-Kōketsu is on another level compared to other Kamui. However, Junketsu is the same as Senketsu, but he never talks. Is Senketsu different from other Kamui?
Senketsu is the only one Kamui that can talk or communicate. Senketsu can communicate only with Ryuuko. This is because Senketu was made by Dr. Matoi with some modification, i.e Dr. Matoi used Ryuuko's Central Nervous System's DNA with Senketu's life fibers, which is why Ryuuko only can communicate with Senketsu and wear it.
After the end of the battle in episode 17, around 6:00, there is a conversation between Ryuuko Matoi and Aikuro Mikisugi:

A : Dr. Matoi made further improvements and completed Kamui Senketsu. Senketsu's life fiber have been spliced with your central nervous system's DNA
R : What ?
A : That is why he can only communicate with and worn by you.

A is Aikuro Mikisugi, R is Matoi Ryuuko. The exact phrasing might depend on the subtitle set.
Ryuuko's Central Nervous System's DNA is the main reason why Senketsu can talk but other Kamui cannot.

But Satsuki was able to wear it and use it. The reason may be that they are siblings. Satsuki can hear and communicate with Senketsu. They are siblings so their DNA match to some extent, and Senketsu contains Ryuuko's DNA. This may be the reason but this is more what I think than a confirmation of existing information.