Part of the reason was that Vocaloid 3 was released this year. This added new languages (spanish, korean), on top of this studis rushed to release vocals. Some of these projects had not been released earlier because they were intended for vocaloid 2, however, for one reason or another, failed to make the deadline for the engine version. Thus they were instead released for Vocaloid 3. Futhermore, several new studios began producing vocals.
In comparison to the start of Vocaloid 2 there were only 3 studios at time of release (Zero-G, PowerFX and Crypton Future Media) and two of them were English vocaloid making studios, which take a while to produce. Internet co., Ltd entered a year later with Gakupo. Most of the current studios entered halfway through Vocaloid 2s lifespan.
Vocaloid had an even worst start then Vocaloid 2, with vocals taking up to 2 years to make in some cases and focus remaining on bringing the engine up to snuff rather then making vocals.