Ilm ul zaahir (outward knowledge) is appealing for a number of reasons including the fact that such knowledge is tangible and stimulates the mind e.g. learning Arabic, fiqh and tafseer. Ilm ul baatin (inner, spiritual knowledge) does not have the same appeal for someone who is interesting in learning and thus some people actually deny it exists.
Ilm ul zahir is shifted away from the individual, for example, when studying incidents about other people. On the other hand, with ilm ul baatin the focus is on the individual and not on others. A person is therefore forced to reflect upon himself.
[This short excerpt is based on a brothers' majlis (gathering held for spiritual training) held by Shaykh Abu Yusuf Riyadh ul Haq on 15th May 2011].