We all know that housing inventory remains at record-low levels; why is that the case, though?

The other day, we reported that housing inventory increased 4.7 percent in March to a 5.2-month supply. Though technically an increase, that represents just a 3.1 percent uptick from a year ago, when inventory stood at a 4.7-month supply. So inventory, in other words, is still quite low.

What gives? For months now, we’ve been waiting for housing inventory to take off and push housing back into recovery mode, but it’s pretty much been dawdling – and thanks to some new research by Redfin, we now have a pretty good idea as to why.

Forty-nine percent of homeowners in the current housing market, Redfin argues, are in a good position to sell their homes; the other 51 percent, though, are addled by one of four problems:

1. Low Equity – Even with all the progress that home prices have made the last couple years, a large number of homeowners (19 percent nationwide, in fact) still owe more than 80 percent of their homes value, and as a result are unable to list the property.

2. Locked-in Rates – We’ve written about the dreaded rate lock-in before, the period of time when, with interest rates on the rise, homeowners opt to stay in their home and enjoy their historically low interest rates, rather than list their property and purchase a new home at a higher mortgage rate. Sixteen percent of homeowners, Redfin finds, are locked-in with low rates.

3. Company/Investor Owned – A nice share of properties (3 percent nationwide) are still owned by either an investor or a company of investors, and as such cannot be listed on the open market.

4. Purchased/Refinanced in Past Seven Years – Simply, the homeowner has either owned the home for less than seven years or refinanced their mortgage in the past seven years; on average, homeowners sell their homes after more than seven years of residency, so it’s unlikely that the 14 percent of homeowners that fall into this camp will list their properties.

As you can see, the reasons are varied for why inventory remains so low, and they impact the nation’s various housing markets in different ways. Check out our graph below for an idea of how those causes disparately affect the nation: