Because of the Old Guard who are the decisions makers in such positions. It's why Vietnam had such a focus on "strategic" bombing, because many came from a schooling and bomber command led by those who often had familial or aircraft based ties to particular schoolings. Those in command were often those who had been taught by either WW2 survivors, or taught by those who had been.
However, at the same time, focusing on immediate threats and Cost/effectiveness ratio isn't everything, as it still needs to provide jobs for someone, and mothballing an entire fleet because railgun technology is still not feasible enough (and contrary to popular belief, throwing money at something isn't always going to work; look at the British Armies focus on the weekend warriors is working out - i.e. it isn't) to consider doing.
Railgun technology is still something that's viable, but doesn't have the best part of 60 years of engineering history behind it with a similar sort of budget, and something that's only just come from outside of science fiction novels for the majority of some people, there isn't a dedicated cadre of engineers; compare how many people go "Oh, I'm going to be an AET", compared to "Oh, I'm going to become a Railgun engineer".
It's a trickle effect, and provided it works in the upcoming conflicts of the next 20 years, there'll be some interest in it.
One thing that has been suggested is that planes wouldn't be half as effective or successful had there been no second world war. It condensed a vast majority of intellectuals together and forced them to achieve particular goals within a tight timeframe. I'm not suggesting another major conflict is needed, but to verify the effectiveness or a particular bit of hardware requires major testing and a secondary design process to back it up in a support function.
Another example is look how long its taken to get UAVs on task. I'm kind of sad that I'll not be able to participate in ops involving British UAV's. They've looked like awesome kit, especially now that things like tablets are becoming more and more commonplace.
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