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Nice work, I am in the same boat as you right now, need to step up to guiding to get longer stubs and beat down the noise.
The reason high ISO doesn't help is because you cannot change the sensitivity of your sensor, quite literally you are just changing gain - the conversion of number of photons to the brightness of a pixel in your image.
The noise comes in because photons arrive at the pixels in a Poisson distribution, so for N photons you will have a standard deviation of square-root of N. Thus low N values have low signal to noise ratio. Doubling ISO also doubles noise and so cannot bring out more detail.
There are a few caveats to this, but it is in general why you need longer stubs or bigger scopes.
Paradoxically, if you are using a canon DSLR, going to higher iso can reduce the noise adding during the sensors reading process (called read noise).
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