Steel vs. Aluminum Wheels
Steel makes for a heavier rim; the modern-box section design works well for taking advantage of aluminum's lower density.  Furthermore, since steel will rust, the rims are typically chrome-plated.  The chrome plating makes rim brakes almost useless in the wet, which is exacerbated by the poor pads used on most of the classic bikes that shipped with steel wheels. In contrast, steel rims are almost indestructible - they can be bent out of shape and re-bent repeatedly and will wear much longer than aluminum rims.

As far as your specific case, many older bike-boom era bicycles were built with "Schwinn"-sized wheels, ISO 590 or ISO 597, both labeled as 26 x 1 3/8".  As far as I know no manufacturer makes aluminum rims in that size, so finding a replacement wheel for an older bike usually means either getting new brakes and converting it to a modern size (ISO 559 or ISO 584) or finding an old wheel that's not too beat up.  The latter is usually pretty easy, cheap bikes ship with steel wheels that are often simply discarded.