Speeches tend to have scripts or at least detailed outlines, whereas lectures depend on the order of information for their organization. The reason: speeches depend on specific words chosen to move and persuade the audience; lectures depend on information for their impact, and the actual words that convey that information can be improvised.
Speeches rely on persuasive techniques that may include not only information, but emotional pleas and earnestness for their impact. A lecturer doesn't try to persuade the audience; rather, a lecturer only wishes to inform them. The lecturer usually has some expertise she shares, while a speaker shares conviction.
Speaches seek to get the audience to agree with the speaker's point of view, while lectures tend to give listeners information they can use to make up their minds. The aim of a speech is to persuade others to choose one option, while a lecture clarifies what options are available.
At the end of a speech, the audience members should feel they know and like the speaker. The speeker is one of them. At the end of a lecture, the audience members may find it irrelevant whether they liked the lecturer, but they appreciate the new understanding they have reached.