ATMEGA328 interrupt pin - why can't interrupt on "HIGH"?

There a several modes to attach an interrupt according to Arduio.cc:

attachInterrupt(0, pin2_isr, XXXX);

LOW to trigger the interrupt whenever the pin is low, CHANGE to trigger the interrupt whenever the pin changes value RISING to trigger when the pin goes from low to high, FALLING for when the pin goes from high to low.

How come HIGH isn't an option?

Why would you want continuous interrupts? That’s not what interrupts are for…

How come HIGH isn't an option?

Because it isn't an option in the chip. The designers at Atmel did not consider it important enough to include that option, and in truth it can easily be got round. Just put an inverter on the input pin and there you are. The interrupt on logic change is much more useful anyway.

arusr: There a several modes to attach an interrupt according to Arduio.cc:

attachInterrupt(0, pin2_isr, XXXX);

LOW to trigger the interrupt whenever the pin is low, CHANGE to trigger the interrupt whenever the pin changes value RISING to trigger when the pin goes from low to high, FALLING for when the pin goes from high to low.

How come HIGH isn't an option?

Because RISING is a better choice, meaning the interrupt will be generated when the signal changes from LOW to HIGH.

Traditionally, the interrupt pin was connected to a pull-up and driven by a number of open-collector (TTL) sources in a "wired-or" (active low) configuration because that is how you implemented a common interrupt line on a computer bus.

So it was indeed, "active-low". "Active high" is simply not used. Since you should not be wiring for example, switches to pull high, this should never be a problem, but note of course, what retrolefty says.