Because what matters to you for a mobile browser doesn't matter to another person.

Because there are measured differences in the speed of Fennec of devices of the same type that might have different software installed.

Perception, that is, what people perceive to meet their expectations based on some preformed opinion, is selective, else we'd all look the same, act the same, and use the same devices in the same way.

MicroB is a good mobile browser for many people; but because of issues related to actual use, the opinions of it as a great browser (aka the personal analysis of the application based on a single user's experience) will differ. The reason this is so is because while the technology of the browser is great, the implementation did not follow use and perceptions by those louder users.

Technically, MicroB is the benchmark to all other Internet Tablet browsers. It should be the worst thing experienced considering what development its not seen in the public eye.