User blog comment:Serza5/English Vocaloids - Their Boom?/@comment-5665682-20140717184825/@comment-3467648-20140718022703

Actually I think is a combination of product design, marketing, and good luck. If you know your target you can achieve decent good results, specially if you have a good quality product. That would explain the success of OLIVER or MAIKA.

IA become popular for two reasons. It was a good quality product with a vocal range and type of voice which didn't have too much competition (realist, with a ballad or opera-esque)  and because it was boosted by Kagerou Project. After the initial boom of that song series, IA Project began to support Jin's newborn franchise which lead to a sucessful result for both parties.

P.S.: BTW, I wouldn't say that MAIKA hasn't been marketed. She has been involved in  3 contest, one of them related to a famous band.