As a Zerg player I feel as though the Zerg race has very few different viable tech options. You pretty much rely on infestors for anti air exclusively. Corruptors are good but lack range therefore forcing them over enemy army's to be killed and they don't do enough damage to kill Phoenix harass or muta harass. Infestors do their job better because they can't.
They are also much more cost efficient than hydras. Hydras cost 100min and 50gas for an 80hp unit, when you can make a slightly less hp and damage unit from infestors and use them when you need them making them easily deployable. Hydras are not deployable as they are slower than molasses in snow.
I think the biggest issue is Hydra HP, it's too easy to lose such an expensive unit to justify making them. Collosus will tear them apart, hellions, banelings, siege tanks, etc. too many hard counters for hydras mainly because they are so squishy.
Another reason no one makes them is because they aren't near as fun as infestors. There is nothing more than A-moving with them. It would be fun if they had a some sort of ability- for example Marauders have concussive shell which adds a fun dynamic to them- not saying they should get that but maybe something similar.
Here's some ideas what I think would make Zerg a little more balanced and army compositions a little more interesting.
- Move Infestor to Hive tech making the Hydralisk den the building required for upgrade to Hive.
-Give Hydralisks 100hp (up from 80)
-Give Hydralisks a slight speed buff ( not an upgrade)
-Make some sort of ability for hydras researchable after Hive tech
Ideas: armor piercing spines; units attacked by Hydralisks have armor reduced by 1 ( just as an idea- feel free to add your own)