Banners have few sizes and standards. Think of 6.3 second TV or radio commercials or odd sizes for print publications. Yet on the web we have everything from rectangles to squares to full-page ads. No two sites are alike it seems. This makes it harder for ad creators obviously, since they have to create different banners for different sites.
Banners are only useful on pages where users actually visit.
It seems elementary, but many sites have little-seen pages, or have traffic patterns that are hard to predict. This often results in many sites having pages that don't have banners on them, what our industry amusingly calls unsold inventory. So placing banners means a solid understanding of traffic patterns by a media buyer. That is wishful thinking, as many media buyers are young with little technical background. Taken together, these trends make for tough times ahead for banners.