The other big factor is that pipes only work because of the power of plain text. The output of every UNIX command is ASCII text, and most of the time, it's ASCII text with columns delimited by \t and rows delimited by \n. And there are command-line utilities like awk, cut, sed, & xargs for parsing & rearranging that text, and funneling it into formats that other commands understand.
The web has a variety of other content types - images, videos, applications, structured data - that don't map well to this model. Before you have interoperable webapps, you need to define common data formats for them to interop.
