We are issuing more long-term debt, but not that much more. We can argue about the reasons—whether the government has done the maximum or could do even more—but the issue with long-term maturities is that there's only limited demand for that.
If the government tries to issue all of its debt as 30-year bond maturities, it will probably have to pay significantly higher interest rates, because there's limited demand for these very long maturities. That may explain why the government has not issued only 30-year bonds but has instead gone, for still a majority of its financing, through shorter-term bonds.