This one should be pretty easy: because In Ears are awesome.
Ok. I’ll be more thorough:
They help you hear better: Everyone has had the experience of playing a show with bad monitors and unbearable stage volume. The whole gig goes by in a blur and you go to bed with the sweet sounds of Tinnitus to lull you to sleep. IEM’s reduce hearing fatigue significantly. They also allow you to hear what everyone is doing regardless of where you are on stage.
They help you sound better: When you are isolated using IEM’s, you aren’t forced to use stage volume to compensate for a bad mix. You can turn your amps down. You can even get rid of amps altogether. Your singer can turn him/her self up so that they don’t feel the need to oversing and wear themselves out. You will be able to harmonize better because you’ll have more clarity and better mixes of yourself and your bandmates.
They help you play better: Playing guitar and singing with IEM’s was an eye opener for me. It really removed any pretense I had about my performance. It is clear and unforgiving. You will hear every mistake made with crystal clear precision. It was a great motivator for me as a musician to improve myself. On the flip side it also allowed the entire band to hear the click track the drummer was using. That allowed us to add cues and other tools to help us perform the songs tightly as a unit. It made a huge difference
They are cheaper than you think: The main complaint from people is that switching to IEM’s can be costly, and they’re not wrong. Custom in ears can be staggeringly expensive. They’re also not necessary. The majority of musicians today are using universal-fit earbuds. Any you know what? They’re great! The other component are the transmitters. The model I prefer is the Sennheiser ew300IEM2 G3. They retail for $1299. That one transmitter allows for 2 mono mixes; which puts the per-player cost at $650. If you are in the market for powered monitors, a QSC k10 retails for $699. Adding those up per player can get pricey. Not to mention the space they take up. From that standpoint it’s actually more affordable.
It’s easy to just point at big acts and say “you should do it because they do” but in this case it’s true. Some times things are popular because they are the superior solution. I have been using In Ears for the past 10 years at this point. With digital mixers it’s never been easier to run them. The time is now!