Why do parasitic elements (e.g capacitance) appear on only high frequencies?
Shunt stray capacitances are problematic when their impedance is too low, as opposite to being too high, because they pull down signal lines and/or mismatches them. 

When the stray capacitance value is much lower than the frequency, then the resulting impedance may be high enough so as not to heavily impair a line. Everything might be reasonably under control. 

However, as frequency goes up this may cease to be true, and then the resulting impedance may start to be low enough so as to pull down the line.