I don't think making clothing for V3 is more difficult as for V3. It is just that people started buying for V4 instead of V3 (next new thing) that killed off that market. Having the remainder get into clearance or vault did not help.

You still can use dynamic clothing on V3. It is often just a matter of "growing the cloth" into the V3 shape. The convsersion tools from V4 to V3 work reasonably well except for fine detailed clothing which take a lot of work to convert