IA had incredibly heavy marketing early on, but she took off naturally much later. The difference b/t Mayu and IA is that IA's marketing succeeded, while Mayu's did not. IA became popular, Mayu did not.
Also, Mayu being on EVERY Vocalocreation album definitely wasn't a coincidence.
In Mayu's case of course not, but in every vocalo- album after her release, she more or less ended up with one solo track at
best, later on it would just be a part of the expected group song really... Despite being made by
the label for vocaloid releases, a lot of her marketing muscle was used for pre-release and was all but dropped post-release.
For IA tho... she had heavyweight marketing and a very large amount of money pushing her to heights a lot of other vocaloids could only dream of reaching. It doesn't really feel like a coincidence to me that she was able to succeed as a result of such aggressive (and still to this day!) marketing. I can't imagine her being able to do so well on her own though.
A big example of being able to on simply it's own merits is GUMI. Whenever Internet Co tries to push things outside of just voicebanks, they tend to flop (concerts, her game, etc), and yet she consistently is in 2nd place to Miku.
That is an example of something making it without commissions imo. Yukari is another example because it took her until nearly 2015 to have a song that reached the heights of the so-called big 8 (or 9 if we include IA).