Because they are free to post their views and concerns all over the place, including Facebook. The fact that someone chooses to post a question does not mean that they are being silenced if the questionee doesn't choose to reply, or indeed if they reply with a stock answer.
If people are that insistent on getting a direct response, they can go to the MP's constituency surgery, they can put questions at public meetings; if they are valid questions, they will no doubt get asked on other forums, like TV interviews and Question Time.
People were constantly asking the likes of Johnson about his lies in Parliament and elsewhere, he and his supporters deflected them every time. Did you complain then that they were suppressing the right to free speech?