The reason is simple: the fixed number of progenitors of any given rank and the fact that there are no open spots among those from the 1st to 3rd rank (meaning, we know all the names, and Aschera isn’t one of them).
There is 1 first progenitor (name unknown, but not Aschera for sure, because of what said in chapter 4 of vol.2 of the Michaela novel); there are supposed to be 2 second progenitors, and we’ve seen them both - Saitou (although he changes his name every now and then) and Woold Gilles; and there are 3 third progenitors, and we know their names - Lest Kerr, Krul Tepes and Nyx Parthe (an interesting thing with her: in the fanbook her name is Gabel Phathema; in the magazine publication it was changed to Gabel Parthe; and in the volume version it was changed yet again to Nyx Parthe).
Starting from the 4th rank, we know no names or faces, so based on the above, Aschera was a 4th rank at best, imo. The fact that he is Krul’s older brother doesn’t mean much except that he’s about as old as her.
What determines one’s strength is the blood of one’s sire + one’s own talents/abilities + the number of years lived as a vampire. The latter doesn’t matter upon turning, so only the first 2 factors are in play. If he and Krul were turned by the same vampire (had to be the 1st progenitor), the fact that Aschera ended up a rank or two behind Krul only shows that Krul was more talented as human, and that determined the resulting strength difference.
However, the possibilities aren’t limited to them both being turned by the same vampire. Only Krul could have been turned by the 1st progenitor, and then she, not wanting to part with her brother or, say, accidentally almost killing him by draining him, unable to resist the bloodlust, had to give him her own blood to save him, in which case the best he would be ranked is the 8th rank (the number of ranks the blood weakens from 2nd gen to 3rd is at least 5, as seen with the calculation of what Crowley’s rank would be had he actually gotten the blood from Ferid in the novel, so 3+5=8th rank, at best).