Make them exempt from property tax and people will build them everywhere. Tho individual shelters are not very efficient and often too weak to withstand a bomb. Compare to the commercial ones which are located deep underground, with a several feet thick blast door in solid steel. If you're in a city with a metro system, that's probably the safest place to be when the nuke strikes. Many of those are specifically built to also serve as a bomb shelter. Pyongyang, Moscow and Stockholm are some great examples.