Well there's a few reasons, some which apply to me: Why people didn't upgrade anything under 1.5? no idea, since I don't think any of those apply to upgrading anything under 1.5
Some people wonder why users haven't upgraded to FF 2.0.
Nubmer one: Firefox 2.0 wasn't an automatic update, and it never asked if you wanted to update. A regular user would have to stumble on FF's website to know that there's a new version. Only now that 1.5 is no longer supported (or possibly a bit before, since I think it was a bit preparatory) are users getting informational messages that they should upgrade their browser.
Number two: 2.0 offers nothing really new. I upgraded to 1.5 quickly, Because as far as I remember, it was one of the awesome-est versions of FF. It added more support for many new web technologies, such as SVG and CSS.
Number three: 2.0 changes some things visually. I like the current look, and I'd have to waste a bit of my time to revert back to the old appearances and layouts.
Number four: 'if it ain't broke don't fix it' -not something I follow much myself, but I do to an extent, especially on old/'fragile' systems. "My current browser version works fine, why should I risk trying a newer version that may cause a conflict with my system, or some other problem?". This is definately the case for some other things like Windows Media Player and Windows Vista!
Number five: Some extensions become no longer supported. This may be hackable, but it may lead to bugs, and the more complicated ones probably aren't hackable. Also most users don't know about the hacking anyways.
Well there's a few reasons, some which apply to me:
Why people didn't upgrade anything under 1.5? no idea, since I don't think any of those apply to upgrading anything under 1.5