Why ground wall insulation is thicker than conductor insulation?

No matter what level of voltage you are dealing with, the ground wall insulation will be heavier than the conductor insulation. Potential to the ground wall is line voltage. Turn to turn voltage, especially in a medium or high voltage machine in minimal in comparison to the line voltage. The coils for medium and high voltage machines are made with the turns in a perfect stack unlike a random wound coil where it is very possible that the first turn of a coil can make contact with the last turn of a coil.. The turns of one coil can make contact with the turns of another coil.

The only other area where higher voltages are a factor is phase to phase. In medium and high voltage machines, phase to phase insulation is not used because the entire coil is insulated at or close to ground wall levels. In low voltage three phase motors a phase to phase insulation is inserted between groups of coils because phase voltage will be a minimum of 58% of line voltage and line voltage when there is a delta connection.

The high potential test on a new winding is twice the working voltage plus 1000-volts. Rewind any motor with insulation the same thickness as the conductor insulation and try that test. The definition of a volt is "a unit of electrical pressure". The higher the voltage the more pressure on the insulation. The higher the pressure the more insulation required.

The purpose of insulation is to keep materials at different potentials from becoming the same potential.

All coils have one or more of the following. The order is in increasing distance from the conductor, which essentially means it is protecting against a higher voltage gradient.
1. Strand insulation - used to protect individual strands. This can be a film, a tape, or a combination of the two. Voltages between individual strands are quite low (less than a few volts, usually, regardless of applied line voltage) and is therefore the insulation thickness (build) is the least.

2. Turn insulation - used to protect one turn from another. A turn may have many parallel strands, all at a similar potential. The average potential from one turn to another is approximately line voltage divided by the number of turns. Since the progression is not linear, which is why the first turn or so may have "extra" insulation added.

3. Phase insulation - used in lower voltage machines that are typically random wound (lots of strands bundled together to form a turn, but without true "turn" insulation). This usually takes the form of separators between coil groups, as noted by Graeme above. Phase voltages for wye (star) connections are (line voltage divided by 1.732); for delta connections, the line and phase voltage is identical. Larger form wound coils (ie.. discretely insulated coils, generally rectangular in shape) do not use phase insulation; rather, they rely on the coil insulation as protection.

4. Coil insulation - also referred to as "groundwall". This is used for the largest potential difference ... the line voltage of the coil to the ground potential of the slot. Because it is the highest potential, it requires the heaviest insulation build. As a side note, it is also generally constructed as the mechanical protection for the coil during the coil manufacture and winding/handling process, which may add some material as well.

5. Gradient insulation - this is usually in the form of a tape or conductive paint and is applied to machines where the line voltage is in excess of 6000 V. This material is because the slot portion of the coil is often covered with a conductive tape to ensure good contact with the ground plane of the slot. A sharp boundary between line and ground is definitely undesirable so the coil is wrapped in successively fewer thickness of the "gradient" material so that at some point out on the nose of the coil the potential is once again at line voltage.