I would suppose the other than 75% are either clueless or think voter fraud is just fine.
Just some wild guesses...
1. people will be intimidated by the process and when asked for an ID. I presented my drivers license last time I “walk-in” voted. I have voted mail-in for several years since then. Seems like mail-in has a bigger risk of fraud.
2. people are pretty irresponsible and may forget to get an ID.
The idea — voting should be as simple and convenient as possible. OTOH: there is the risk of cheating.
Here in California a state-issued I.D. costs $26 and they have a special program for poor people so it only costs them $7 to get an I.D.
http://www.dmv.ca.gov/dl/fees/idCard_fees.htm#reg_id