Why might quantitative researchers be regarded as insufficient?

Because numbers can sometimes describe:

how often something occurred

without adequately explaining:

what it meant to the people involved.

But is that judgment arbitrary?

Not necessarily.

If the research question is:

“How many people supported X?”

statistics may be highly appropriate.

If the question is:

“What did X mean to people in 1850?”

textual and contextual analysis may be more appropriate.

So methodological adequacy depends partly on:

whether the method can actually answer the research question.