The VC simply doesn't have the time to review an entire business plan. From the entrepreneur's standpoint, however, it is quite difficult to boil down all the key information about their company into a 2-3 page executive summary. We must be careful to note that the executive summary is considered fine for an initial contact, but investors still expect a full business plan to be available should they express further interest. The preference for e-mail contact may be a factor in the investors choosing to see an executive summary initially rather than a full business plan. Most business plans would have to be e-mailed as attachments.
Attachments require time to download and print out.
The executive summary simply could be included as part of the initial contact e-mail with no attachments necessary. The popularity of e-mail combined with the preference for an executive summary may be an indication that the VC prefers a more efficient review process.