The "Interface" number listed in an addon's TOC file doesn't mean "this addon is only compatible with this version of WoW". Instead, it's meant to be a quick way to determine "when this addon was last updated". Most patches don't change anything that most addons need to be updated for. I know I'm not the only person around here still using addons that haven't been updated since the WoW 1.x days.
Some addon authors do post "updates" on patch day that don't do anything besides change one or two digits in their TOC file, to tell WoW the addon was updated. As an addon author myself, I feel that it's a waste of my time to change a few digits and upload a whole new version of my addon when nothing really changed. And as someone who uses about 125 addons, I feel that it's an even bigger waste of my time to download and install all of those non-updates every time there's a WoW patch. And last but not least, I'm sure that Curse and WoWInterface would prefer not to have to pay for the bandwidth consumed by all the users downloading all these non-updates. :)