So it is ok for an at-will to be just weaker than others, because there is a specialized build (Pyromancer) that benefits more from it's damage type?
But using frost-cheese (lasting frost combo) isn't good enough and available for everyone, so Freezing Burst can be better.
Still not convinced their is a rational for the weak Scorching burst besides they avoid errata in this case.
The rationale is that they don't errata to power things up. They errata to remove the most broken things, or things that aren't working correctly. There are a few exceptions (careful attack, and some instances where higher level encounter powers ended up being worse that lower level powers, etc)
If a power is broken, it will see play, because it's probaby so good that quite a few people will use it, and thus it will disrupt the game.
If a power is underpowered, it probably won't see play. So, unless it's something a character is forced to take, it's not in PRESSING need of errata, since it won't be a problem in play. And, if it's only RELATIVELY underpowered ... like say any non-twin strike at-will for the ranger ... there is probably not as big of a problem. Yes, the cold burst power is better than the fire power, but is scorching burst worthless? Not necessarily. There is no reason to pick it over the cold one UNLESS you want fire damage (for pyromancy, or you know you will be fighting trolls, etc) but if you did pick the power, it's not like your character is necessarily going to be gimped.
There are a number of other classes with encounter powers where you have comparable powers at the same level where one is purely better than the others ... in this case, there just happens to be a reason that someone would want the lesser option, because they want it to do fire damage.
But, long story short, it wasn't errata'd because it was an "ok" power that just happened to be obsoleted by a newer power, but is still useful if someone needs it for the keyword. That's more that can be said for some other at-wills that have no redeeming qualities at all.
If they tried to make every at-will power balance to each other equally they'd constantly be issuing errata. Sometimes it's just worth it to leave suboptimal (in a vaccum) options there because there may be some reasons to take them. They did, sort of, obsolete thunderwave, but the thunder keyword does mean you can grab the paragon feat and increase the burst (and if you crank wisdom you get a bit farther push).