I don't think that these hacks are entirely the reason behind the fall in the price of bitcoin but they definitely contribute. I think mostly they hold back the price because they scare investors from investing in to cryptocurrencies for fear of being on the end of one of these such hacks. Once security and regulation is stricter than it is currently some of these investors may feel safe to enter the market.
I agree with what you are saying. Hackers stealing and selling Ethereum has definitely led to a drop in the value of Ethereum which has affected the entire market directly, but this is simply one of the reasons why Bitcoin is falling in value. The other major reasons include the prolonged bear market this year which is leading people to think that 2018 is the reverse of 2017 scaring off many investors and forcing current investors to cash out their profits as quickly as possible. The bear market will not continue indefinitely and I am expecting a reversal by August this year.
There have been many tokens and hacks it's just not reasonable to think they're entirely the reason. The amounts alone aren't close to being enough to put such a dent in the market given that the market cap is so high. They probably don't force many people in to selling because most in crypto know the risks, it's just they put off others.