I wouldn't characterize the history that way. First, there are multiple reasons that SDI was not pursued, beyond concerns around its feasibility. Most notably,

I wouldn't characterize the history that way. First, there are multiple reasons that SDI was not pursued, beyond concerns around its feasibility. Most notably, bilateral agreements were signed with the Soviet Union to cease development of ballistic missile defense technology. Those agreements held until only very recently, and they seem to have been effective in mitigating the worst of the Cold War by both preserving the status quo around nuclear deterrence and heading off another extremely expensive large-scale development program. Both the US and the USSR agreed to only field two BMD systems and actually fielded only one at most during the Cold War; SDI just didn't fit into the political/military climate of the time and there were good reasons not to push that envelope. Besides, the extreme cost of SDI meant that there was enormous opportunity cost. One of the criticisms of SDI is that it was such a sink of money and political capital that it delayed the development of other approaches to missile defense and broader nuclear security. Second, BMD efforts in no way stopped cold after the failure of SDI. SDI survives to the present day, after a couple of renamings as the Missile Defense Agency. MDA continued to develop many of the more practical aspects of SDI and these serve as the basis of BMD countermeasures today. The problem of BMD is very challenging considering adversarial countermeasures and even modern systems show poor efficacy, but that's not to say that MDA efforts haven't produced outcomes. Major improvements in fire control technology came out of the program, perhaps most significantly the development of phased-array and electronically scanned radar, and attendant broad improvements in microwave electronics.