The short, wide copper blade would not hold an edge and was soft, so that it could only be used at short range to hack and stab. Bronze is brittle, as is cast iron, therefore blades made of these materials would break easily when they were used for blocking. The longer the sword the longer the effective fighting range, so that the full array of fundamental techniques in use today were only made possible with the discovery of hardened and tempered steel in this millennium.
Point 1, Copper was used for razors in the old and new world, it can hold an edge like steel, Just because our modern culture can't get it to do so does not mean the Chinese and Andean and Egyptian people couldn't get it to do so, and indeed steel like edge qualities have been observed in some copper blades of these cultures, and indeed the manufacturing process is not understood.
Point 2, In a televised test of a Bronze sword and a Steel sword, where they were struck into each other at high speed, they sustained equal damage. Bronze is not always more brittle than steel, as an allow it can vary a great deal and old bronze blades are amazingly sharp and strong. Bronze weighs much more than steel and is harder to differentially harden, thus steel is superior for handling and manufacturing purposes, but not actual durability!
Point 3, Some Chinese bronze swords that are rather long, evidently the Chinese could make long strong sharp bronze swords of a quality that defies modern understandings. Here is an example:
http://www.philosophyblog.com.au/images ... 500bc1.jpg
There are a few other examples, some of which are surprisingly long for bronze swords.
So much knowledge was lost in China about this that it is very sad. They made possibly the finest bronze swords on earth, the quality of which is seldom matched by the finest of steel swords new or old. Though actually it was not really China, which is a nation, not an identity, so when you go back several thousand years you don't actually stay in the same culture, which does explain why there was no continuity of technological information.