I have a couple questions regarding current motherboards.

Are the VRMs linear or switching regulators ?
What supply voltage does the VRMs use to derive the 1.5V ?
Why no 1.5V output on ATX supplies ?

I understand if we use higher voltage we can use less current and therefore smaller gauge wiring but is that the main reason for the dependence on the 12 volt rail these days ?

Thanks for any insight.

12V is because you can make all others from it, except the negative voltages. VRM's are powered by 12V too (see the 4 or 8-pin connector pinout).

The Pentium Pro was the last Intel CPU to use a fixed voltage (3.3V). Starting with the Pentium II the motherboard is responsible for the CPU power supply. Originally motherboards used 5V as a source, later 12V. Ideally the PC power supply should transition to 12V only.

All modern motherboards use PWM-based VRM components that draw their input from 12V. The main variation among motherboards is the number of "phases" (separate VRM circuits that timeshare the CPU voltage output), and quality of capacitors (electrolytic on cheap boards and "solid state" on high-end boards).

As mentioned above, recent CPUs have almost individual Vcore requirements; the CPU tells the motherboard its preferred voltage using pins called VID (voltage identification). AMD has shipped CPU models that were genuinely customized after packaging, while Intel sets the VID with each new stepping or process change.

Midrange and high-end motherboards let the user override VID in the BIOS to overvolt or undervolt the CPU, either to overclock it or run it cooler/greener.

And yes, one reason for all this is wiring. A high-end CPU can draw 100+ amps of Vcore (100+ watts at ~1V). Trying to provide this from the main supply would not only require remarkably stout wires, the voltage would be highly unstable because of the resistance and inductance of the wires. This is why on all modern motherboards the CPU socket is literally surrounded by VRM MOSFETs and capacitors, not to mention several layers of heavy copper inside the board attached to hundreds of power and ground pins/pads.