People about Cosplay...
I’m going to address this question from a different viewpoint. Why is cosplay popular amongst those that do it?
There is the factor of honoring the source material/fandom that you love. There is the creative outlet. Then there is the sharing of the work with other fans and making new connections with like-minded people. Getting complimented for your work can create a sense of accomplishment, as can having a photographer ask to take your picture. A three day event like a local comic-con or Anime/SF/Fantasy Convention can be more rewarding than a two week vacation in the tropics, to some people.
This all has a spillover effect, those that aren’t doing it, see the results and fun that those that are having and want to do it themselves next time. This keeps the world of Cosplayers growing at a currently staggering amount. I’m staff on a 2,200 person Anime Convention and we are constantly amazed at the ~60% cosplay attendance.
For me, cosplay is the art of bringing a fictional character to life using yourself as a medium. As people have already mentioned, cosplayers put a lot of money, time, and energy into their craft. Some will go to extremes to lose weight or learn a technique outside their skill level just to try and cosplay that character.
I really love cosplay and I find it is a mirror of how you will treat a lot of things in life, whether that is your job, your relationships, etc. The typical time and project management questions always appear (“Can I finish this in time for this convention?”, “Do I have the resources?”, etc.) but also some that you may not expect (“Should I just stitch this poorly to save time?”, “Will cutting a corner here affect the project later on?”, “Should I invest the time in learning this new technique and apply it to the costume?”). The way you answer these questions are exactly the same way you will answer these questions in real life.
That’s why cosplay is such a great way to make friends than other activities. Not only do you have a great excuse to geek out to like-minded people about your passions, you also can see whether or not they do as they say. You really get to know the people and can truly bond with them.
It depends on what you mean by popular. It’s more or less exclusive to conventions, which have the benefit of only happening once each year per con. Attendees like myself have time to craft and sew awesome things. I know why I like it-it pushes my creative boundaries and has become my full time hobby and side-job. It’s very fun to be someone else, but it’s also fun to show off your work. I made my husband a great Groot costume for DragonCon last year and it was one of the best weekends ever. We had a blast as ‘Rockette’ and Groot. There’s something about participating in a huge fandom that is just pure fun.
I don’t think it’s really all that popular with the general public, when people don’t know who you are it can be disappointing. They don’t really know what to make of you when you come into a restaurant painted blue with a handful of storm troopers after an event or convention. Kids seem to go nuts over it though, I’m also a member of Rebel Legion and dress as Princess Leia for events. Adults are amused but kids go crazy for Darth Vader. Popularity really only seems to stretch as far as known characters.