so....

Currently, Windows is our area of speciality. Making portable apps for Linux is quite different to making portable apps for Windows; the diversity causes some quite significant problems, so in some regions it's considerably harder. As for your certainty that at least half the users are Linux users - that's not at all true.

Since Linux is open source why aren't all the Linux users developing their own version of portable apps?

It seems like most Linux users have no technical skills.

As much as I like linux, I see no need for portable linux apps here, as almost every other computer I encounter uses Windows (or maybe OS X). Would be nice to carry my apps around with no machine to run them...
I rather use live-cds / Boot-USB drives for that purpose.

on linux, most distros come with wine today, and most apps here will work fine under wine. Therefore no need to to port them .

The problem with linux is, every distro is different, has different set of libraries and apps for linux do seldom contain the needed libs with them. Instead dependency lists are created and the missing libs have to be either manually or automatically installed form the distros repository. Under those circumstances it is very difficult if not impossible in some cases to make portable apps for linux, since in most cases one will either need to carry all dependencies on the stick with the app, or install the required parts on the host system, thus changing the host system in fact.

Or do you expect to have a portable app in versions for ubuntu, kubuntu, knoppix, suse, fedora, slax, DSL, puppy .......?