The main reason being that the rental market is saturated with flats and competition for tenants is high, landlords offer lots of reduced rents, plasma TVs etc to get tenants in. Once they're in they usually stay for 6 months and then move onto the next place offering freebies. Also young professional tenenats like to be in the newest trendiest complex, yours maybe "it" now, in 6 months time another will be "it". We ended up with flats being rented for only 9 months of the year, and couldnt make money this way.
You are quite right about this sort of behaviour. Tenants are quite fussy and demanding. You get some tenants that just look for refurbished properties and then move on to the next place. Would be nice if tenants stayed on in the same place for longer....