The answers I hear mostly are;
"I don't want to get injured".
MY RESPONSE: Injuries are part of running at a high level.
" _____________ (famous gifted runner) never ran doubles and look what he/she did"
MY RESPONSE: __________ was one in a million. You are not.
" I don't run doubles and I'm improving, why should I?
MY RESPONSE: Your window of opportunity is short. You are undermining yourself and not maximizing your potential.
I'm sure there are other better excuses out there. Let's hear them.