Final answer:
A small fraction of extrasolar planets are detectable by the transit method because it has inherent limitations that make it easier to detect large and close planets, while observational biases make it difficult to find small or Earth-sized planets with longer orbital periods.
Step-by-step explanation:
The reason why only a small fraction of extrasolar planets are detectable by the transit method is due to inherent limitations of the transit method (c). This indirect detection technique requires a precise alignment where the planet passes directly between its star and us and thus causes dips in the star's brightness. However, such transits are only observable for a small percentage of exoplanets due to the vast number of orbital planes in which they could be aligned.
Additionally, observational biases make it easier to find large planets that create significant brightness dips and those with short orbital periods because they transit more frequently within the observing intervals of telescopes like Kepler. Since we are likely to miss long-period planets, it may take longer than the duration of a mission such as Kepler's operation to observe enough transits to confirm an Earth-like planet with a one-year orbital period.
Moreover, the observational capabilities can be an issue, especially when trying to find smaller Earth-sized planets, which have a much subtler effect on the starlight and are therefore harder to detect compared to larger planets. It is not that smaller, Earth-like planets do not exist; it is that current detection methods have an observational bias towards bigger and closer planets.