I would suspect that whether direct attack spells were conjuration or evocation comes down to making sure each school had some rather evenly, split up amongst the different energy types. The same way they've tried to make sure there were spells of all the different energy types at as many of the levels as possible (to help accommodate balance amongst the different Draconic Sorcerer bloodlines for example), they also seem to want to make sure Evocation doesn't get overpowered by assigning it every single direct damage spell, as would normally seem to be the case if you were to do a more scientific reclassification of where spells fell into which schools.
I mean truth be told... if wizards in-game truly did a "scientific" classification of all the spells they know into certain "schools"... I'm pretty sure there would end up being much more than the eight the D&D game uses. We're talking real life animal and plant levels of classification-- high level Kingdoms down through Phylum, Domain, Genus, Species and the like. Magic would be classified that same way. Spells that grant positive life energy would be split off from spells that attack with negative energy... spells that use a particular energy type would be grouped together... conjurations would be separate from summonings... illusions that directly affect a person's mind would be separate from graphic illusions in the environment... etc. etc. etc.
At the end of the day, I think WotC did what they did just so that they could maintain consistency with the tropes of the past, while also keeping schools relatively balanced whenever possible. They wanted to allow for players to play an "Illusionist" like they could back in the AD&D days... so they continued the school system and fudged things so that it could remain useful and worthwhile.