The short answer is that the 30-year mortgage amortizes extremely slowly, making it nearly twice as risky as a similar loan with a 20-year term. And the 30-year loan compounds risk-layering by promoting the use of higher combined loan-to-value and debt-to-income ratios (DTI).
The Housing Lobby unabashedly supports the broad availability of the 30-year mortgage, and even wants it extended to manufactured housing. This is because Housing Lobby sees the slow amortization as a feature that reduces monthly payments, making home “more affordable”. They choose to ignore the bug–the 30-year loan, when combined with other risk factors, drives up home prices when the supply of homes is tight, especially for buyers of entry-level homes. Since 2012, lower priced entry-level homes have risen by about 55%, while move-up homes have risen by about 31%. This means lower priced entry-level homes that cost an average of $103,315 in 2012, cost a whopping $160,138 in late-2018. Thus, rather than making housing more affordable as its supporters claim, 30-year loans make housing less affordable. But more on this entry-level home pricing penalty later.