The same reason any critique of Thompson is met with choruses of "Fear the Fred!" It avoids having to actually address the criticism by passing it off as unworthy envy or panic.
Nobody I have seen who supports either Fred Thompson or Ron Paul has failed to address criticism of their candidates. In fact I have seen many such supporters reveal such criticism to be frivolous.
Fact is that at the grassroots level Ron Paul is a phenomenon and at the primary level Fred Thompson is widening his lead.
All the former ‘tiered’ candidate supporters get cross-eyed when they see the success of Ron Paul and Fred Thompson so they start making stuff up to criticize them with. When their arguments are shown again and again to be weak, they ignore the rebuttals and keep on repeating the same tripe arguments. For example, you will hear them say Fred Thompson was prochoice when he never was. Their evidence is weak, nonconclusive and out of context. It doesn’t matter to them. So why do they persist? Because they fear their guy/gal will lose.
So yes, it’s about fear and fright. It’s not about avoiding the issues.