I remember ETH being at $90 USD and now it's at $175 USD however still gas price is still at 20 gwei? Can anyone explain this?
In theory miners set their gas price based on a trade-off between more transactions per block producing higher revenue and the greater orphan rate making it more likely that their block will be orphaned. Vitalik discusses this in detail in this post.
Since the block reward that you lose out on if your block is orphaned is set in ETH, in theory it may well be rational to leave your gas price at the same ETH rate, even though those ETH are worth more USD. If you drop your gas price to fit the increased value of the fees you will generate more fee income, but you also lose more in block rewards.
However as of 2017-05-25, miners gas price settings do not appear to vary much, so it may be that miners are simply not paying much attention to them. There is more information about the current state of gas prices at gasstation.info.