Land-based ICBM forces can cost significantly less money to procure and maintain than an equivalent force of SLBMs or bomber-launched standoff missiles. Also,

Land-based ICBM forces can cost significantly less money to procure and maintain than an equivalent force of SLBMs or bomber-launched standoff missiles. Also, both SLBMs and bomber-launched weapons are vulnerable to tactical countermeasures. Fast-attack submarines do routinely shadow patrolling ballestic missile subs, with a credible intent of attacking and destroying the missile subs if a wartime launch appears imminent. Both bombers and air-launched standoff weapons are vulnerable to interceptor fighters and ground-based SAMs... Whereas nobody has really managed to crack the problem of successfully intercepting a mass MIRVed ICBM attack in the terminal flight phase. But the biggest issue is that we never really know whether our enemies have developed some new capability (be it technological, procedural, or intelligence-derived) that would critically compromise the effectiveness of a major weapons system. If we put all our eggs in one basket, we become more vulnerable to a catastrophic, war-losing breakdown like that.