How come RS doesn't get it?

RS is shown as having the kind of bond with Simi where each understands the other's feelings, even unsaid. Why is RS not puzzled by Simi's reluctance to commit to the Nakul rishta and her obvious lack of enthu or pleasure when the topic comes up.

RS definitely knows and understands Simi. He understand that there is some reluctance from Simi's side. He senses that very well, but he doesn't know what exactly it is.
RS might be thinking that with RB fiasco, Simi's heart is 'broken' and hence he knows he is not supposed to rush things with her.
Even though Manju is shown grieving over her son, Anuj, she very well knows that she is a mother of 3 unmarried daughters. Its like a 6th sense with mothers, that will always be working and she is the one who is in hurry to marry her daughters off to any 'male'!! A family with an eligible bachelor coming and asking for the alliance in itself is a BIG thing for her! So, she will behave exactly that way.
RS does see Simi is holding up and hence he talks to her in the kitchen that night. He assures her once again that it is her life and she can take all the time and he would trust her decision whatever it may be. But he also gently reminds her that it would not be good to keep Nakul's family waiting forever.