Why are some instruments like the USD/RUB and/or USD/CNH disabled on occasion?

Both of these stated markets are considered exotic. Because of this, at times, there is very low liquidity in the market. During those times, the spread on the instrument may be very high (the spread is variable so it can change) or the instrument might be fully disabled from trading. This can happen with other markets but is typically unlikely. With these considerations in mind it is imperative that any trader factor this into any trading decision.