Nickel Bullion?

I think that it's mostly tradition. Copper is one of the traditional coinage metals, along with gold and silver, while nickel wasn't purified until the late 18th century. Copper also has an unusual color for a metal, but the silvery-gray color of nickel is really generic. Silver coins and nickel coins don't really look all that different from each other at first glance, but copper coins are immediately different from them.

One problem with nickel, as with other potential "industrial bullion" metals such as titanium, is that the cost to actually shape and strike a round becomes a significant component of the total cost, compared to the scrap value of the metal it contains. Copper, silver and gold are all ductile metals with low melting points - so they're cheaply and easily shaped into rounds, ingots, coins and whatnot. Nickel has a very high melting point in comparison (1455 deg C, compared with copper at just 1084 deg C) and is very, very tough in terms of impact resistance (which is why Canadian pure nickel coins have such a low relief), so to strike a round in the conventional fashion, you'd need very high die pressures and/or a very unattractive shallow relief design. High temperatures and pressures means high energy, which means high production costs. That cost has to be recouped somewhere. Would you be prepared to pay double the "bullion content" of a pure nickel round, just to own a pure nickel round?

Titanium has an even higher melting point (1668 deg C) and is even tougher, which is why nobody's even tried making circulation titanium coins yet, let alone "investor rounds".

It is much more difficult to get temperatures that high in a home forge; you'd be hard-pressed trying to find one that'll go much above 1300 deg C. You pretty much need a large-scale scrap metal refinery to make pure nickel recycling cost-effective, and is another reason why "bullion rounds" of hard-to-process metals such as nickel make little sense. If you need a piece of metal for an aesthetic or industrial application, a silver round is relatively easy to convert into the form you need. A nickel round is not.

Of course, using the same logic, a platinum round shouldn't make much sense either, and there are plenty of those about. I suppose in that instance its all about rarity and perception of value. Whoever said the human race had to be "logical" about these things?

The cheapest way to alter the form of a big chunk of nickel is to dissolve it in acid (unlike silver and gold, virtually any strong acid will dissolve nickel, so you can use whatever acid is cheap an readily available) and re-precipitate it where you need it in the form of a thin nickel plating. Most "cheap" modern world coins are made of nickel-plated-steel these days. But making an entire pure nickel ingot or round by this method is neither cheap nor simple.