In M31, the galaxy is almost edge-on to us, and there isn't enough contrast between the arms and the spaces between so they appear to merge together into an oval. In M51, although fainter, the projection is perpendicular to the line of sight, allowing us to plainly distinguish between the bright spiral arms and the darker spaces between them.

Another factor in visual observing is we tend to have our attention attracted to the nuclei of galaxies, which are much brighter than their spiral arms. Most people, for example, don't realize how big M31 is, almost three degrees across, the size of Orion's belt! Instead, we focus only on the bright inner core.