The "measure of electability" you call arbitrary, most people call "didn't get enough votes to make a difference to the election". The party you support hasn't made enough inroads with the vast electorate to have any noticeable influence on national elections. That won't change until your message is accepted by a higher percentage of the voting public. Don't blame us Republicans if you can't win enough of us over from even the mediocre candidates that have been foisted off on us in past years; our candidates are still getting more votes than yours - and it's your fault.
Taking that a bit further, I believe we can identify some traits that may keep Libertarians from being more successful, more than fringe. It's not libertarianism itself. It's hard to disagree with a lot of the libertarian contingent's views on liberty and the constitution. I'd like to see the Republican Party lean much further in that direction.
The sticking point is more the Libertarians conclusion. For all their rightness of thought on Liberty, they conclude with the puritan, all or nothing, black or white, us or them. I'm not sure I'd want to live in a community where that kind of candidate is electable. A Republic of millions just can't be only one thing or another and nothing in between.
I'm not saying I want compromise that will further a destructive direction. We need something like an economic triage. I want a problem solver elected with a more prioritized focus to recognize and address the most immediate needs first, the fortitude to make the tough decisions, and the political savvy to make it stick.