Many of us have been playing the games for years. We have a lot invested in our existing characters. Aside from any emotional attachments we have based on history and memories, we have special items, many of which are no longer available, our old tier pieces, our full banks kitted out with bags and goodies. We have investments in professions, we have heirlooms, we have supporting alts, we have all our gold.
To most people, it is not at all appealing to start on a new server and not only restart their whole gaming history with no heirlooms, no gold, no alts to make bags or glyphs or gear but also have to reinvest all that time they spent leveling and gearing their character (and any alts they may also want).
Can one save the $25 and do it? Sure. Is it an appealing option for people who can't swing the costs? Not at all. Most people would probably rather just quit the game if they couldn't afford a transfer, or else stay on their current server and play unhappily until the game stops being fun and they quit that way.
Server transfers are popular for a reason.