Why does Orwell object to ready-made phrases and mixed metaphors?

Orwell believes that ready-made phrases and mixed metaphors cloud meaning for both the reader and the writer and should thus be avoided.

Ready-made phrases are those segments of writing that have been set in motion by someone else. Generally speaking, they have a pleasing sound, but Orwell cautions that falling into the trap of using others' phrasing prevents writers from deeply considering their own writing. Good writers, he argues, deeply consider each sentence they construct, first examining what they want to say and then intentionally crafting sentences that best convey that intended meaning.

By using ready-made phrases, writers skip this deep reflection and may cloud their intended meaning, even from themselves. Orwell uses phrases such as "bloodstained tyranny," "free peoples of the world," "lay the foundations," and "achieve a radical transformation" as examples of such phrases that writers and speakers often employ. Relying on the phrasing of others, he contends, turns writers and speakers into "robots" who exhibit a reduced state of consciousness.

Mixed metaphors reflect this same thoughtless construction of ideas. Orwell points to writers who use phrases such as "Fascist octopus has sung its swan song" to demonstrate this mental cacophony of ideas. Instead of making the ideas more clear through the use of figurative language, writers who employ such metaphors demonstrate that they aren't really thinking about the topic at all. Mixed metaphors indicate that a writer has a general idea of what he wants to say but then convolute the meaning like "tea leaves blocking a sink."

Both types of writing, Orwell asserts, indicate that a writer has decided on an emotional meaning for a piece of writing but is not interested in the details of communicating a clear message.

Why does George Orwell condemn the use of "ready-made phrases" in "Politics and the English Language"?

In his essay "Politics and the English Language," Orwell points out several common writing practices that lead, in his mind, not only to bad writing but also to poor and even dangerous thinking. His point is that our use of language affects the way we think. 

One of the errors Orwell points out is stringing together cliched phrases. He writes:

"As I have tried to show, modern writing at its worst does not consist in picking out words for the sake of their meaning and inventing images in order to make the meaning clearer. It consists in gumming together long strips of words which have already been set in order by someone else, and making the results presentable by sheer humbug. The attraction of this way of writing is that it is easy."

In other words, people don't choose words carefully and with a deliberate consideration of their meaning. Instead, they use well-worn phrases that they borrow from others. The attraction of this type of writing is that it is simple; however, as Orwell writes, the result is unclear writing: "By using stale metaphors, similes, and idioms, you save much mental effort, at the cost of leaving your meaning vague, not only for your reader but for yourself." If the writer uses cliched phrases, she (or he) saves herself (or himself) from the problem of thinking and deciding what she (or he) means. As Orwell expresses later in this essay, the result of using cliches is that the writer can try to obscure or hide his or her meaning and even try to defend concepts, such as warfare, that are evil or malicious in intent. Unclear writing allows the writer to express evil ideas without seeming overtly evil.