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To elaborate on red16v's comments, it's all down to the shadowmask and matching spacing of the sets of three phosphor stripes (or dots in older sets) on the crt. When the triad of colour stripes measures the same width as what it is trying to display, all you will get is an unreadable splodge on the screen.
A monochrome monitor has no shadowmask and the electron beam gets directly onto the screen. This gives an ever-sharper image as the size of the screen gets smaller.
A colour monitor gives an ever sharper image as the size of the screen reduces, but only up to the point that the shadowmask starts interfering with the definition it is trying to display. Roughly speaking, an "average" tube will display this crossover point at around 14".
Higher resolution colour tubes would be much more expensive, as they have a much finer shadowmask, which makes them both easier to damage by knocking, and trickier to manufacture to get the purity true.
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