I'll let NBC tell you why:
The three largest voting manufacturing companies — Election Systems &Software, Dominion Voting Systems and Hart InterCivic — have acknowledged they all put modems in some of their tabulators and scanners. The reason? So that unofficial election results can more quickly be relayed to the public. Those modems connect to cell phone networks, which, in turn, are connected to the internet.
The largest manufacturer of voting machines, ES&S, told NBC News their systems are protected by firewalls and are not on the “public internet.” But both Skoglund and Andrew Appel, a Princeton computer science professor and expert on elections, said such firewalls can and have been breached.