Whereas the web is unified by the HTTP protocol, P2P has no dominant protocol. The strongest driver of this trend is HTTP itself, which has too many strengths to ignore but too many weaknesses to use exclusively.
The greatest strengths of HTTP are that its design is mature, there are implementations for virtually all working environments, and it is the only protocol able to travel through most corporate firewalls. The greatest weaknesses are that it is slow and unreliable, has many features that make no sense outside of web browsing, and is exclusively client/server in design. To get around some of these problems a number of new protocols have arisen.
The introduction of new protocols has introduced a new problem - that specific content is generally available only through one of the many protocols. For example, one is more likely to find a file by searching both the Napster and Gnutella networks. Because no one protocol can address all these problems, it has become common for a single program to support multiple protocols.
It is an open question whether this situation will continue once P2P is mature.
See also Two Way Web.