It is because while your wife's income is excluded for the purposes of determining your taxable income, it is not excluded for the purposes of determining the tax rate on your income. In other words, based on your income alone, your tax rate is 10%. When your wife's income is included, your tax rate is 25%, which is the rate at which you'd be taxed with an additional $94,000 of income.
It is likely that this is one of the rare instances in which you'd be better off filing separately, because then the tax rate at which your income alone would be taxed is much lower, and since you wife is a non-resident alien she would owe no US tax anyway.
