Zoom the chart in as much as possible. Bid & ask are so close together on higher time-frames they appear to be 1 line. ***Both charts below are H4.
I think the above explanation by BKR88 sums it all, as the difference is just a few points, that is why we do not see the ask line on the higher timeframes. Mostly it is visible in M1, M5 or M15.
You may often not see this line also because of your broker's minimal spreads. After all, the spread is the difference between the purchase price (ask) and the sale price (bid) of a currency pair. And the smaller the spread for a currency pair, the closer to each other the ask and bid lines will be.
An ask line may only be enabled in a small time-frame because it reflects immediate market conditions, showing the most recent price at which sellers are willing to sell. In shorter time frames, price movements are more volatile and frequent, making the ask line more relevant. Longer time frames might average out these fluctuations, reducing its significance.