。
History is generally soft on tradition, you know.
Literal
History [topic-は] inevitably tradition [on-に] is-lenient so [informative-よね].
どうしても ('inevitably, no matter what, can't help but') — here in the 'it can't be helped that X' sense rather than its common 'absolutely must' sense. に甘い ('soft/lenient toward') is a stock collocation: 甘い ('sweet') metaphorically = 'indulgent, lax, lenient.' The ~からね at the end is an explanatory 'you see.'