"If this guy has been a suspect from long ago why havn't they gotten a dna sample from him a lot earlier."
Because he was not a suspect until recently.
I have been following this case for more than a couple decades. The one thing that BTK was very good at was making sure that the possibilities of who did it were in the thousands.
There was a lot of speculation for a lot of years that he may have even been one of the people investigating the murders, because he always seemed to be one step ahead of the police.
He did a very good job at making sure the fingers did not point to him.
The "smart" folks at the Wichita Police Department even looked into this guy in the 1980's and weren't brilliant enough to put the pieces together and declare him a suspect until today.