You've hit on a couple of really good points that are rooted in the history, cost, and standardization (or lack thereof) in electrical wiring. Let's break it down

Why Black and White for NM-B:
Historical Reasons

The use of black and white for NM-B (Romex) wire is a holdover from the earliest days of electrical wiring. Black was designated as the hot (ungrounded) conductor, and white as the neutral (grounded) conductor. This system was established primarily for 120V circuits, which were (and still are) the most common. While 240V circuits existed, they were less prevalent.

Cost & Simplicity

Manufacturing NM-B with only black and white conductors keeps production costs down. It's simpler and cheaper to produce cable with just two standard colored wires. Adding more color options would increase inventory and potentially slow down the manufacturing process. The economies of scale for high volume black/white NM-B are substantial.

Industry Inertia

Changing established standards is a slow and often contentious process. The electrical industry tends to be conservative, and there would need to be a strong consensus for change to overcome the existing conventions.

Code Flexibility (for 240V)

The National Electrical Code (NEC) does allow you to re-identify the white wire as a hot conductor for 240V circuits using colored tape or other approved means. This makes the existing black/white NM-B somewhat versatile. The NEC doesn't require specific wire colors for 240V circuits, as long as the white conductor is re-identified.

Why Not More Color-Coded NM-B?

You're right, it does seem like a logical extension to offer pre-colored NM-B. Here's why it likely hasn't taken off: Inventory & Distribution Challenges

Adding more color options significantly increases the number of SKUs (stock keeping units) that manufacturers, distributors, and contractors need to manage. This creates inventory holding costs, logistical complexity, and the risk of obsolescence. Consider how many different gauges, conductor counts, and lengths of NM-B already exist. Adding a half dozen color combinations would dramatically increase this complexity.

Lack of Strong Demand

While some electricians would appreciate pre-colored NM-B, the overall demand might not be strong enough to justify the added production and inventory costs. Many electricians are accustomed to using tape to re-identify conductors, which is an acceptable (and relatively inexpensive) solution.

Standardization Conflicts

Introducing more color codes for NM-B could create confusion and potential safety issues if not widely adopted and standardized. The current black/white system is well-established, and changes could lead to errors, especially among less experienced electricians.

Competition from Other Wiring Methods

For more complex wiring scenarios, many electricians prefer to use conduit with individual conductors. Conduit offers more flexibility in terms of wire color, type, and future modifications. NM-B is primarily used for simpler branch circuits.*

Market Size vs. Cost

The market for specialized color combinations in NM-B might simply be too small to justify the investment. Manufacturers focus on the highest-volume products to maximize profitability.

In summary: The electrical industry tends to be resistant to change, particularly if that change is not broadly applicable and has limited gains. The manufacturers make the products, so there would need to be a significant demand by electricians, driven by efficiency and/or labor cost savings in order to motivate a large-scale change of this nature. Flag for review