Legacy servers are populated with folk who played pre-ARR 1.0 FFXIV. It was ridiculously easy to get MASS AMOUNTS of Gil during end-game through leve abuse amongst other things. Enough to where Yoshi had to reduce peoples Gil down to 10% of what they originally had (we're talking billions) for the launch of ARR.

So while YOU may only have 30k, other Legacy players can easily have hundreds of millions of Gil. This explains the disparity in housing prices between servers. Allowing them to transfer would be a huge headache. Example: If they were allowed to transfer to one of the "low-tier" servers where houses only cost 6mil, they could easily buy up all the plots as just one example.