Is there a reason why there are 7 Good summons, and 19 Evil summons?
Because when the spells were designed, there were far more evil creatures in the bestiary and a very small number of good creatures.
Is there a reason why there are 7 Good summons, and 19 Evil summons?
Because the others are still in 3.5 (BoED for one adds like 6).
In general when a monster book comes out people complain about all the useless good aligned monsters in the book, after it came out people complain about the lack of summons available for good aligned casters.
I rather see more specific summoning spells, the lack of flexibility can be compensated by having slightly more powerful summons. The spells are thematically more interesting that way and makes it easier for players to keep some statistics on hand for commonly summoned creatures rather than a bestiary full of choices, the planar binding and ally spells could do with a little boost instead of being useful in niche cases for creatures capable of teleportation and the like, being much more interesting than the somewhat boring and flavorless summoning spells. Just my opinion ofcourse.
I prefer my playrs to summon the same creatures over and over again. That way nothing whacky slips past me, I give em some leeway on what they can summon. Each new spell level gets a new creature, or a lower level creature plus that creatures buddies.
They (the players) are more invested that way. Summons don't end up as trap springers and cannon fodder either.