http://heroichollywood.com/home-1/2015/6/23/an-inside-look-into-tom-hollands-spider-man-casting
This sounds like the most probable reason Asa wasn't cast.
The truth is whatever discussion Marvel and Sony had over the casting of Spider-Man, that deliberation ended after the screen-testing weekend back on May 30th in Atlanta’s Pinewood Studios. See, Asa Butterfield and Tom Holland were the frontrunners all along. While Marvel and Sony certainly weren’t happy with Asa Butterfield talking about the role on Reddit, that wasn’t the defining factor that made Tom Holland our new Spider-Man. The truth is that both parties of Marvel and Sony were very impressed with Holland’s audition, like Devin Faraci had reported some weeks before the screen testing, but there’s more.
Who was the actor that flew to Atlanta that same Saturday to help with the screen testings? That’s Robert Downey Jr.. As everyone knows, Downey has a reputation for his talent, charisma in his stunning portrayal of Tony Stark. However he is also known for being relatively short, standing at 5ft 8in tall to be precise. Can you picture the skinny Asa Butterfield (6’0’’) screen testing with Robert Downey Jr. (5’8’’)? Tom Holland in the other hand is shorter than Robert Downey Jr. (5'7''). While that wasn't the main reason that Holland got the role instead of Butterfield, it certainly didn't hurt Tom's case to look good next to the Marvel Cinematic Universe's main man. Back on June 3rd, when Asa Butterfield posted on twitter #6ftclub four days after the screen-testing happened, he already knew that Marvel and Sony wouldn’t need him on set back in Atlanta. This wouldn't surprise me at all, You cqn't have someone tower of the highest paid actor in your group.