There is a reason Guitar Hero appears more popular than Rock Band / 2.
The answer is quite simply the approach each company takes.
Harmonix has gone the route of weekly DLC, every single week since RB1 was released and lately we've been getting as many as 10 to 13 songs per week. Everyone who is planning on buying Rock Band most likely already has it and a large chunk of them are updating it each week with new content.
Activision/Neversoft has gone the route of one new title per platform every 3 months.
The new "Guitar Hero 5" is actually the 13th release in the GH series, with GH: Van Halen to be the 14th.
You can't buy these things online, and each one comes with its own instruments, accessories, etc. It's no wonder you see more GH at stores and no wonder they move so much more through retailers.
The GH titles to date:
1. Guitar Hero 1 (Harmonix)
2. Guitar Hero 2 (Harmonix)
3. Guitar Hero: Rocks the 80's (Harmonix)
4. Guitar Hero 3 (Neversoft takes over from here on)
5. Guitar Hero: World Tour
6. Guitar Hero: Aerosmith
7. Guitar Hero: Metallica
8. Guitar Hero: On Tour (NinDS)
9. Guitar Hero: On Tour 2 (or whatever it's called, NinDS)
10. Guitar Hero: Handheld (Stupid little handheld battery operated thing)
11. Guitar Hero: Arcade (The arcade version of GH3 that you see in malls and is never calibrated right)
12. Guitar Hero: Smash Hits
13. Guitar Hero 5
14. Guitar Hero: Van Halen
See what I mean?
For comparison, here is the rock band list of releases
1. Rock Band 1
2. Rock Band 2
3. Rock Band: Unplugged (PSP)
4. The Beatles: Rock Band
Unreleased: Lego Rock Band
GH3 and RB1 came out within a month of each other.
Think the series is sufficiently milked yet?