When fighting with Cumber, Vegito used Kaioken to increase his power. But now Vegeta has shown he can use Super Saiyan Blue Evolution at will in Dragon Ball Heroes. Since Kaioken stress the user body, why Vegito chose to use kaioken and not Super Saiyan Blue Evolution when fighting with Cumber? It was stated that for the canon main continuity, Super Saiyan Blue Kaioken x20 is equally strong to Super Saiyan Blue Evolution transformation
There isn't any particular reason, however, the only thing which makes logical sense is the fact that:
- SSBE was designed to be a Vegeta exclusive form similar to Ultra Instinct which as of now seems to be a Goku exclusive form or even SSJ3 which as of now is only a Goku exclusive form.
- We haven't witnessed a Goku+Vegeta fusion utilizing the SSJ3 form.
- Kaioken is a technique and similar to instant transmission which the fusions are capable of using. So it would make more sense for them to use techniques instead of a character exclusive form.
- Based on my previous point, it would be also interesting to note that based on the nature of Kaioken used with Super Saiyan Blue, ideally, it should be stackable even on top of SSBE as the form is simply an evolved state of Super Saiyan Blue. Which only leads to the conclusion that the fusion was utilizing Kaioken simply as a technique during the course of the fight.
This may be attributable to the mindset of the fusion vs. the mindset of the form.
Goku's UI is, as the name indicates, a pure instinct based form that doesn't rely on active strategy or emotion. It is noted that it's pretty inimical to Vegeta, who is a very non-instinctual and cerebral, yet still emotional fighter. So a fusion that combines both Goku and Vegeta seems unlikely to access UI.
SSBE is the flipside to Goku's UI, utilizing Vegeta's more cerebral and emotive nature and eschewing the calm, instinctual nature used for UI. For much the same reasons that UI is unlikely to be used, SSBE is also probably hard to access: half of the fusion is just intrinsically unsuited to the transformation.
Kaioken, however, is a technique based on energy control. Both Vegeta and Goku had to drastically improve their energy control in order to make SSB alone more efficient and potent. They remark at various points that SSB is very much a form that is all about energy control. As such the Kaioken technique is well-suited to the fusion, as both the Vegeta and Goku halves have both extensively trained their energy control.