5. NOT UNDERSTANDING THE LOCAL CLUB SCENE
A city's musical venues represent a sort of ecosystem. Bars and clubs that book local bands come and go, with a handful seeming to be "permanent" fixtures. Unless your band is made up of members who have been around a long time and played in a bunch of other popular groups over the years, it can be difficult to get your foot in the door at a lot of places. After all, why should a club take a chance on a new and unproven band? All illusions about "helping the scene" aside, most venues are in the business to make money, and if a band doesn't seem like it can draw a crowd, that club probably isn't going to give them much of a shot. Fortunately, most cities tend to have a few venues that will give beginning bands a chance; these are worth seeking out instead of griping that the fancy venue down the street that features national touring acts won't return your calls.
4. PISSING OFF THE WRONG PEOPLE
Of course, if some band members are generally considered to be assholes, that doesn't usually make networking go smoothly. No one is going to want to help your band if he remembers that the singer is that guy who got caught trying to steal a club's microphone, or if someone in your group is a creep who treats other people like shit. Going onto social media to bitch about how a certain club "sucks" (especially when it doesn't), or to tear into other local bands, is just dumb. People remember that stuff, and it doesn't make them want to help you. I've known some musicians with personalities so toxic that they've eventually just moved out of town because they'd burned so many bridges. Having someone like that in your band probably isn't going to rocket your band to success.
3. COPYING THE WORKS OF OTHERS
I've noticed a weird dynamic with bands over the years. Not every band is going to be wholly original, and a lot of them tend to follow currently popular musical trends rather than trying to come up with something completely new. And that can work sometimes. If it were 1992 and your band sounded "almost like Soundgarden” when no one else in town did, then that could have been enough to make you stand out and draw a crowd. However, if it's a few years into a trend and lots of other local bands are playing the same type of music, it gets harder to get noticed. In the long run, it's fine to play a style of music that's growing in popularity, but at some point a band needs to find a unique sound of its own, or a variation on a trend, if it wants to really shine.
2. LACK OF A GOOD SONGWRITER
I can't count how many times I've gone to see a show and the band is made up of virtuosic players who play as tightly as is possible, but the songs are boring. I'm not sure why this is, but I think a lot of musicians spend more time practicing the technical skills necessary to play great, but either can't or don't spend much time learning to write songs well. That's fine if you plan on only playing covers, but anyone who wants to roll out originals needs to practice writing songs, too. There's a reason a band like AC/DC sells more tickets to shows than, say...Michael Angelo Batio, and it's not because Angus can shred faster.
1. YOUR BAND SUCKS
Let's be blunt here. If you expect your band to succeed beyond playing backyard keggers for free, then you need to reach at least a minimum level of competency. We've all sat through performances that sucked, and we all recognize that experience as not being fun; after a point, it's unreasonable to expect an audience to love a group who doesn't entertain them. A band might not be good for countless reasons – it's not always about musicianship, and most issues can get better with practice. Still, it's probably a good idea for a band to recognize any problems of quality before an audience does it for them. Sometimes songs need to be tweaked, or performances need to be more engaging. Being able to accept constructive criticism is probably a valuable tool for musicians as well; almost every musician has been in a few bad bands at one point or another. There's no shame in that, but it's better to take a hard look at things and try to improve them than to bitch endlessly about how no one "gets" what you're doing. They probably do get it; they just don't like it.
(I apologize for the Celine Dion video, but it's offered as a reminder that being popular isn't the most important thing in the world.)
BONUS: KNOW HOW TO NETWORK (NOT SOCIAL NETWORK)
These days it's possible for any band to promote itself in ways that weren't easy a few years ago. Anyone can get his or her music online, a video on YouTube and a presence on social media. The problem is everyone does that, and it's also easy to get lost in the jumble of other bands promoting themselves in exactly the same ways. Perhaps more important is to network — band members need to attend shows and try to get to know people in other struggling bands, as well as the folks who work at clubs. It's amazing to me how many bands seem to think that they can just promote themselves online, rarely engage with other gigging musicians or go to shows, and then somehow emerge from their bedrooms to play a packed show. Unless you're already some local-music legend, that's not likely to happen. One has to get out and network with people in the real world.