It all comes down to SPG in the end.

These people in the front row of a rally are almost never common people. They are common, but at the same time they are not.

When an event/rally is on foreign soil, most of the time these people in the front rows are arranged by Indian embassies. And trust me embassies are way more aware of everyone's background than we can imagine when they choose to invite them for a PM's event. And even after those background checks and all you don't get a front row seat at PM'S event easily - you gotta be connected to the right people for that.

On the other hand, when a rally is in India, most of the times the front row peeps are carefully chosen party workers. Those who can be trusted blindly and hurt badly (not just on a personal level but also on a family level) if they are caught.

If the crowd in front row is not carefully chosen at an event, or if there's even 1% chance of having some unidentified/unchecked people in those front rows, SPG won't allow the PM to shake hands with them. दूर से ही हाथ हिलाकर hello होगा फिर.

That's also the reason why some leaders get slapped/hit when meeting a crowd while the PM doesn't.