I know why it's ignored. If you're reading this, you might know the answer to that question, too.
But knowing why doesn't change the fact that the verse is ignored, and anybody who dares obey it gets backlash from people who claim them to be unloving and angry.
Before we get into that pesky question of why, let's first look at what the verse says:
But now I have written unto you not to keep company, if any man that is called a brother be a fornicator, or covetous, or an idolater, or a railer, or a drunkard, or an extortioner; with such an one no not to eat.I Corinthians 5:11, KJV
The context is overall about whom we're not to have fellowship with. (It's the context where Paul's telling the Corinthians to "put away from among yourselves that wicked person" [v. 13].)
According to this verse and its context, we're not to even eat with someone who 1. claims to be a Christian and 2. engages in one of those sins. (We'll get to what those sins actually are—or at least what the one is that explains why the verse gets ignored—in a moment.)
This is a "both/and" setup, not an "either/or"—something Paul says explicitly in v. 9–v. 10. We can associate with people who engage in egregious sins—that's what witnessing is! We just mustn't associate with folks who claim to follow Christ yet engage in habitual sin.
Note that "habitual" before "sin". We're all sinners (ref. Romans 3:23), and we'll slip into sin despite our intentions (ref. Romans 7:15, v. 19), but we must not use that reality as an excuse to go on sinning (ref. Romans 6:1, v. 15).
Someone who gets drunk is only a drunkard if they have a habit of drunkenness. That doesn't mean it's okay to be drunk—it isn't (ref. Ephesians 5:18)—but it is a distinction between someone who is drunk (which is a temporary state) and someone who is a drunkard (which describes a habitual pattern of behavior).
Both the temporary state of being drunk and the habitual pattern of being a drunkard can be changed, and a believer will act to change them (ref. Ephesians 5:18).
"Ye shall know them by their fruits" (Matthew 7:16) and "by their fruits ye shall know them" (Matthew 7:20)—and Matthew 7:15–23 makes clear that we are to be mindful of others' fruits. Not merely their words.
Others are to apply that same mindfulness to us—and this mindfulness is to be practiced carefully. As believers, understanding that we are imperfect ourselves and that we will be judged by the standards we use to judge others (ref. Matthew 7:1), we must look for specks in our own eyes before alerting others of two-by-fours sticking out of their own eyes.
But all too often, people instead focus on specks' in others eyes while two-by-fours are sticking out of their own (ref. Matthew 7:1–4), revealing themselves to be hypocrites.
That hypocrisy itself is not unexpected, due to the nature of our hearts (ref. Jeremiah 17:9), and wolves in sheeps' clothing are in the church (ref. Isaiah 29:13, Matthew 15:8, Mark 7:6). So how do we know what we are?
Well, how do we react when someone points out an inconsistency in our own behavior? Or when someone's conviction means they practice I Corinthians 5:11 on us?
Do we assume that one party must be in the wrong, thereby idolizing the opinion of that party? Or do we carefully examine ourselves (ref. I Corinthians 11:31) and the Scriptures (ref. Acts 17:11) to find what merit there is in the words, regardless of source, and then respond according to that merit?
Others have rights to their own convictions and consciences—that's the entire point of the debate about meat sacrificed to idols, described in I Corinthians 8. How they respond to something is on them, not us.*
*No person is responsible for any other person's behavior, though there are situations wherein a person isn't without guilt for another's behavior. If the distinction isn't obvious to you, you've experienced redefinition of words.
It doesn't matter if you scream in my face; you're responsible for your own behavior, and I'm responsible for my response. My response might be justified or not—but that's on me, not you, and you're still responsible for your own behavior. Even if you wouldn't have done it if I hadn't done some other thing.
This brings us to why people ignore I Corinthians 5:11.
See, that word "railer" in the original Greek, loidoros is itself the adjective form of the verb loidoreó—which refers specifically to verbal abuse as a whole (ref. John 9:28, John 9:28, John 9:28, John 9:28).
Our actions are what reveal what we believe (ref. James 2:18), regardless of our words—and regardless if we're aware we believe it or not. Wolves depend on us to pay attention to the sheepskins they wear (the pretty words) and ignore the fruits in their actions that reveal the wolf beneath. So they distort some of God's Word to hide or overwrite other parts of it, ignoring the inconsistency and the violation of the commands to mind others' fruits (ref. Matthew 7:16, v. 20).
I Corinthians 5:11 both tells us to pay attention to others' actions and to act ourselves in response to the fruits of their actions, and that makes wolves uncomfortable. They want to be perceived as sheep, not wolves, so they have a vested interest in dissuading others from paying attention.