It's really odd how there are so few 1.5TB and 2TB hard drives being sold. Here's Newegg.com's breakdown of capacities sold for 3.5-inch SATA internal drives:
500 GB: 16 (cheapest $49.99)
750 GB: 7 (cheapest $69.99)
1TB: 13 (cheapest $79.99)
1.5TB: 2 (cheapest $119.99)
2TB: 2 (cheapest $239.99)

The underlying analysis of your research looks like the market may be reaching the leveling off point similar to what digital cameras are experiencing. Years ago (specifically 1999) when the first 2 megapixel digital cameras were introduced a certain pattern was developed in the industry where megapixel capacities of digital cameras could be somewhat predicted. 2 megapixels then 3, 4, 5 and so on. The pattern began to stretch itself out when the market saw 8 megapixels on a common basis. The reason being is two-fold.

1: The image sensor of cameras are only so large. Cramming more pixel sensors onto the sensor results in a less quality image, especially at higher ISOs. There's only so much space on a hard drive platter to cram in storage. The more data space that is crammed onto the platter, the more likely that sectors of the drive will become corrupted.

2: Companies think they can determine what consumers want. They think most consumers don't need a camera with more than 10 megapixels. They also think that most consumers don't need hard drives larger than 1 TB.

I think hard drive and camera manufacturers rely quite heavily on factor #2 in determining what they produce. Unfortunately companies listen to too many consumers that don't know any better. And as result, mediocrity becomes the standard.

I want a 1.5TB drive for $100. The industry better release more 2TB drives, so it lowers the price of 1.5TB.

Just this weekend Kristina in Chicago was saying how she was gonna use a flash drive as a backup for her files. I nearly fell out of my chair when she said that. I told her to not use flash drives as archives. Goodness. Just look at the name. Flash. That's how your data is stored. This same weekend I just had my 2nd flash card go corrupt on me.

I just asked a question on facebook in my status message, "what size(s) hard drive(s) do you own?"

Here's my response:

I am storing 315 GB at work, and 810 GB at home.

work laptop: 75 GB capacity. Using 55 GB.
work external drive: 1 TB capacity: Using 260 GB.
home computer: 80 GB capcity. Using 60 GB. ... Read More
home external drive: 500 GB capacity. Using 250 GB.
home external drive: 750 GB capacity. Using 500 GB.

I'm currently using:
C: 600 GB [4x 150 GB RAID0]
D: 3 TB [4x 1000 GB RAID5]
E: 1 TB [2x 500 GB RAID0]
Laptop: 500 GB

Looking forward to a price drop as I'm starting to run out of space :/

Nice chart, thanks for it. I've been holding out for cheaper 2TB's myself to build two identical NAS machines.

I have four computers and here are their drive configurations as best I can recall:

HTPC: 120GiB 2.5"
(hey, it's quiet)
Workstation: 2x1TiB, 1x500GiB, 1x128GiB SSD
Laptop: 1x128GiB SSD
NAS: 6x500GiB in RAID6, 1x80GiB IDE boot drive.