It's down to canopy strength, and the F-35 requiring a greater level of bird-strike protection than either the F-16 or F-22. Don't forget, the F-35 has no HUD to protect the pilot's face....
OK.
And I had no idea that the HUD counted toward bird strike protection. The HUD seems like an awfully flimsy piece of hardware to provide bird strike protection. If anything I would have thought a HUD would makes things worse. Thanks for the clarification.