Back in the Biblical times, they had no concept of electronics. All they knew was that the mark was something on or in (depending on Bible translation) the right hand and/or forehead and without it, you could not buy or sell anything. How could you pull that off during Christ's time? If it were a tattoo, you could still be able to make backdoor deals and bypass the system. However, with a microchip implant making electronic payments, the government would be able to know every little thing you purchased and from whom. A lot of crime would be reduced because drug dealers, money launderers, and black marketers would easily get caught.
Some say it could be microchip implants, others say it's an electronic tattoo. Maybe it could be both. Right now it appears microchip implants are the strongest candidate.