Well, in their view it is not a big enough threat to attack and spend millions on gaining a majority of the hashing power.  Once it is determined to be a threat it will be too large for them to buy their way in to control.  Plus the second they gain control is when all of the money floods out of Bitcoin and in to a different crypto.  That is the beauty of it, we are not forced in to using the currency so attacking it results in wasted money as the users move out of Bitcoin and in to a different crypto-currency.