WMG: Kamen Rider Kabuto
The Hopper Zecters were intended for mass production.It would explain why they're so much more streamlined than the others, with no Masked Form, no additional weapons, and the Zecters themselves are reversible so that they can be either Punch Hopper or Kick Hopper as needed. Masked Form, and especially Cast Off, would require a great deal of additional training to use effectively (and safely, especially if you were working in an entire squad of the things), and the same thing with weapons that would be much more powerful than the standard Zectrooper arm cannons. Simplest solution to getting entire squads of Kamen Riders out on the streets, upgrading the Zectroopers to actual Zecters? Strip out all the more complicated systems, and just give them the choice of whether they'd rather punch things or kick things as necessary for the situation. If the plan hadn't gone in a different direction entirely, Zectroopers would probably all have been issued belts and Hopper Zecters.
The Hopper Zecters are the prototypes of the other Zecters.Similar to the theory above of no Masked Form and no additional weapons, but also a Mythology Gag considering that Kamen Riders #1 and #2 are based on grasshoppers also.