Mainly because it was cumbersome (the JTAG hack boots into hacked mode more or less instantly with basic extras while the shader hack took older hardware (even at the time), extra hardware and a minute or two (as well as some fiddling with buttons) to get to. I suppose above all though there were no rebooters so you were stuck playing old games.
JTAG would have probably been even bigger though had it hit before MS closed the hole (although there was warning) and had the hack (plus rebooter) been around for all versions sooner (I recall several threads along the lines of "I am updating, I do not care about such things" only to be banned in a few weeks time).
Timing was a downgrade thing (no efuses were blown this time around so you could download) but most used it to get their CPU keys with which to unban themselves and do fun things like region changing (or coupling with dual nand mods to have both big regions or Japanese if necessary)..
