The lines have been drawn and labels affixed. In comics many believe there to be three distinct consumers: Reader, Collector and The Ones Below All - the Speculators. As the book store channel experiences a boom in graphic novel sales, year-after-year and digital downloads nibble away at comic store sales, revenue spikes from Speculators could mean the difference between life and death of print comics. Below is a graph with data from Comichron to illustrate the importance of Speculator-driven revenue bursts.
One could assume that the sum of copies ordered of issue #1 (84,153) were to service the combined interest of Readers and Collectors. A dramatic drop-off of copies ordered going into issue #2 is normal and from that point on, the interest of the Reader sustains the continuation of the series.
The Immortal Hulk #14 ended on a cliffhanger that suggested Betty Ross was going to transform into one of her former incarnations as the Red She-Hulk or the Harpy. Ewing surprised everyone by introducing the Red Harpy but this didn't occur in issue #15 as expected. Instead, she made a cameo in #16, allowing the speculation conversation to continue through an additional publication cycle, creating more awareness and resulting in the title's peak sales of 90,305 copies ordered by retailers.
Speculators and Collectors were especially interested in character introductions in The Immortal Hulk due to an unprecedented secondary-market value of $100+ for issue #2, first appearance of the Walking Ghost AKA Doctor Frye, a character inspired by Dell Frye from the 1980s TV series. The first mention and appearance of the One-Below-All are two separate books that still command $20+ on eBay.
Anticipation surrounding Rick Jones and the Abomination, coupled with critical acclaim for the series maintained demand until issue #21 when the extreme decline mirrored the dramatic incline.
Why did this happen?
Market saturation proved that yes, there could be too much Immortal Hulk but by no fault of the contents within its pages. As the title marched toward number 20, multiple issues headed into their third, fourth and fifth printings with new gimmicks like "ratio" variants that damaged the exclusivity and integrity of a title that is still a perfect storm of creative forces coming together.