It's because these big companies are afraid to handle Bitcoin's volatility, for any company who will be receiving Bitcoin as a payment they are also risking themselves of losing the value of the payment seconds after they receive it and for any big company they aren't willing to risk their revenues because of it. This is also why Steam , the gaming platform, suspended Bitcoin as their payment option just when Bitcoin suddenly fell during the 2017 bull run their earnings wear hit because of it. I know the solution for this companies is to have some kind of third party where they'll be the ones receiving Bitcoin and in turn this party will be paying them fiat currency which is what they mainly want, this might be their best option right now if they want to open a big door of opportunity for them.