I'll give you a quick rundown, I'm not really a coder but I know the arguments on both sides.
1: It's more work to support Windows XP. The support for Windows XP was removed quite a while ago, and this is one of the first actual complaints about it. Is it really worth the developer's time to maintain and fix bugs on an OS that almost none of the userbase is using?
2: VS2010 was an inferior IDE and was holding back the developers. Switching to VS2012 has made Dolphin faster, it's allowed the developers new debugging and coding techniques to fix bugs that were highly problematic before.
3: D3D9 was holding back graphical development on Dolphin. zFreeze + Tev_Fixes_New will fix hundreds of games with issues when they're complete, and neither can be merged with D3D9 around.
4: On the other hand, Apple users have to remain as updated as possible to run Dolphin. You can ask Pauldachz for details, but being even two or three years out of date with OS updates makes Dolphin not work properly.
5: Dolphin 64bit will run much faster than Dolphin 32bit. Dolphin has been optimized specifically for 32bit. There is literally no reason for someone with a modern processor to be running a 32bit OS. You'll even notice a sizeable speedup on the build that killed Windows XP support; the old OS is holding back Dolphin.
6: The developers took a long time to process this decision, and it was not an easy one. The fact is that no one on the team, even those pro XP, did anything to stop XP support from being removed showed that it was more trouble than it was worth to keep it around.
7: Last but not least; you still have the older builds of Dolphin to run. Dolphin 4.0 was purposefully released with both D3D9 and Windows XP support.
Even though other emulators haven't dropped Windows XP support, it's very likely that if they did, they could gain the same benefits of Dolphin. To them it's not worth it yet, but at a certain point it will be, and more of your favorite programs will no longer run on your computer. Not because your computer is slow, but because your Operating System is outdated and difficult to maintain support for in newer IDEs.
1: It's more work to support Windows XP. The support for Windows XP was removed quite a while ago, and this is one of the first actual complaints about it. Is it really worth the developer's time to maintain and fix bugs on an OS that almost none of the userbase is using?
2: VS2010 was an inferior IDE and was holding back the developers. Switching to VS2012 has made Dolphin faster, it's allowed the developers new debugging and coding techniques to fix bugs that were highly problematic before.
3: D3D9 was holding back graphical development on Dolphin. zFreeze + Tev_Fixes_New will fix hundreds of games with issues when they're complete, and neither can be merged with D3D9 around.
4: On the other hand, Apple users have to remain as updated as possible to run Dolphin. You can ask Pauldachz for details, but being even two or three years out of date with OS updates makes Dolphin not work properly.
5: Dolphin 64bit will run much faster than Dolphin 32bit. Dolphin has been optimized specifically for 32bit. There is literally no reason for someone with a modern processor to be running a 32bit OS. You'll even notice a sizeable speedup on the build that killed Windows XP support; the old OS is holding back Dolphin.
6: The developers took a long time to process this decision, and it was not an easy one. The fact is that no one on the team, even those pro XP, did anything to stop XP support from being removed showed that it was more trouble than it was worth to keep it around.
7: Last but not least; you still have the older builds of Dolphin to run. Dolphin 4.0 was purposefully released with both D3D9 and Windows XP support.
Even though other emulators haven't dropped Windows XP support, it's very likely that if they did, they could gain the same benefits of Dolphin. To them it's not worth it yet, but at a certain point it will be, and more of your favorite programs will no longer run on your computer. Not because your computer is slow, but because your Operating System is outdated and difficult to maintain support for in newer IDEs.