Financing a trade-up move is far more difficult now than it was before the economic meltdown of fall 2007. Today's guidelines require such buyers to qualify to carry two mortgages, to have verifiable income and have enough cash for a downpayment.
Most of today's repeat buyers can't qualify to carry two mortgages at one time. And, they can't come up with a cash downpayment without liquidating the equity in their current home. In most cases, this means they need to sell that home before they can buy the next one.