Down is considerably more difficult to work with for a DIY project. Not impossible, and with some care and consideration more than manageable, but never comes close to the ease of working with synthetic insulation.
Then you have the fact that for warm weather quilts you the fabric becomes a much greater proportion of the overall quilt weight. Not the insulation. For VERY rough numbers, assume a ~1oz/square yard fabric (like finished momentum 90). If you have a 36"x72" (2 sqyd) design you end up with 4 oz of fabric (2sqyd*1oz/sqyd*2 sides). Stitching (especially baffles in down quilts) can add around another ounce of weight. So for a basic quilt you're looking at around 5oz just for the shell material. Now if we use down, we probably only need around 1.5" of loft for a 40F summer quilt (using data off thru-hiker's 17oz down quilt instructions). This gives us around 4.5oz of 900fp down.
Now if we wanted to use synthetic insulation we'd use probably a ~2.5oz/sqyd product like Climashield Apex (in fact this is exactly what MLD uses in their 45F spirit quilt).
So for a synthetic warm weather quilt we're looking at around 10 oz (before bells and whistles like velcro) and 9.5oz for down. However down takes considerably more time and effort to construct which most warrant isn't worth the minor weight advantages.
Now for colder weather, quilts use significantly more insulation. The fabric and stitching becomes a smaller percentage of the total weight. This means a more efficient insulation (ie down) now starts to make a noticeable impact on the quilts finished weight. If using 10oz of 900fp down is the same as 15+oz of synthetic insulaiton (probably more synthetic actually), we're looking at finished weights of 15oz for a down quilt and 20+oz for a synthetic. At these weight discrepancies the extra effort of down has a tangible benefit.
This is why you often see down cold weather quilts and synthetic summer quilts in the DIY threads. The synthetic also has a secondary benefit in that warmer weather tends to be moister in most the country (not in my AZ though), so the synthetic provides some poor weather security as well in above freezing temps that down does not have. Not a major concern for many, but a nice perk for being "lazy" during construction of the quilt.