It's not a matter of being nice but rather a matter of time. The boys are currently recording songs for the second album and are touring right now. They have a lot of work to do and hence there's little time for replying to fan mail.
This is a tip from Carlos (yes, the real Carlos!); if you want an autograph remember to include an already addressed envelope. It's easier for them and saves a lot of time. Also, include your Twitter in the letter and they might tweet back. However, remember how busy the boys are, so don't be disappointed if they don't write/tweet back.
My 5 year old daughter just loves BTR. She wrote them a great letter that was very colorful and decorated and drew them a pic of herself and each of them lol. This was over 3 months ago. I understand they are busy but she was very disappointed. I finally just wrote a letter "from them" which she loved ;-)