I mean, they did try though. Their intention was to take out as many Native Americans as possible. The only thing historians argue is that it wasn't the Europeans who started the spread. Smallpox was already spreading before that, so they just added on to an already growing epidemic.
That's simply inaccurate on two counts. First, it's an extreme generalization to say that "they" wanted to do anything. The European precursor for expansion had dozens or more reasons for doing so, and most had nothing to do with "taking out" Native Americans. Secondly, those that did have blood lust lacked the sophisticated means of intentionally spreading certain diseases. They didn't do it on purpose.