Why did only Stannis come to defend against the Castle Black attack? Didn't Aemon Targaryen tell Sam to make sure that all the crows are fed and to send the word to all the kingdoms?
It was Stannis' Hand Davos that had persauded Stannis to go to Castle Black and save it. As Stannis himself comments
As also stated by Stannis, he knew he had to go to Castle Black
Stannis also had other reasons for going to defend Castle Black
As to why the other professed "Kings" or Wardens didn't send anyone to help Castle Black can be explained as Broklynite states, that they were still fighting and if they weren't fighting then we were trying to rebuild. Everyone kingdom apart from Dorne had been involved in someway or another in the War of the Five Kings. So they needed to recoup their losses, rebuild their towns, castles and tend to their people and lands. Their armies would also have been short of men, therefore instead of sending men to Castle Black to help them, they needed to retain whatever men they had left for their own protection. Also there's a history of some regions, especially in King's landing, of looking down upon the Night's Watch and not really giving it much consideration. As we learn from Yoren (long, long ago) he is sent to retrieve prisoners from the many dungeons in Westeros who have taken the Black, and who are the very lowest and worst of society.
Several reasons that boil down to:
Stannis is a little bit different in the show, but in the books he is portrayed as a rather harsh man who fundamentally believes in duties and responsibilities. That is why he cut off the fingers of a man he trusted at his side, who saved his life. Not out of cruelty, but out of justice.
He decided to try to make it work for him by trying to raise more of an army. Ultimately, however, it was his duty as King, and he took his duties seriously.
Which is why some of the things he did at the end of the last season were rather...shocking, to say the least.
As for why the rest of them didn't come, the rest are people who were too busy fighting to become king or stay as King than to really worry about the lands they supposedly ruled. Which isn't a judgement- that's how civil wars work. They are nasty, vicious things that only end when one person stands triumphant and can get down to ruling. And classically, it sometimes happens that in ignoring those external dangers for the internal ones, they will find the State has been taken out from under them, and all the fighting was for nothing.