One of the by-products when conversion to PEG (at the processing plant) is ethylene glycol (EG).
Think of a fractioning column for petroleum products; the heavier molecular weights (higher viscosity, thicker) stay lower in the column because of their higher melting point -- they boil off, or are drawn off, last. the smaller the hydrocarbon chain, the lower the boiling point to distill out (purify) the end product.
PEG-200 (lightest of the PEG viscosities) has a boiling point roughly equal to (within 3 degrees C@ 760torr) ethylene glycol. EG is the primary constituant of automotive anti-freeze, and as you've heard all your life, is very toxic (sweet to the taste, but deadly). The problem with PEG is it's somewhat more difficult to purify PEG-200 than the other viscosities, requiring a more complicated (and expensive) multi-plate (vacuum) distillation process. This is also one big reason why PEG-200 is difficult to get in NF/USP grades at a reasonable price (in bulk for resale).
PEG-300, on the other hand, is almost as attainable as PEG-400, and is basically the same price. I do not know why it is not being carried.