I think it mostly has to do with the carry over from Classic/Standard... which was explained to me once and made perfect sense back then:
"Think of it as stepping on ants - if you go for the hill, you're likely to get more of them with a single stomp than going after the ones scattered away from the nest"
There's fewer units in a Battle Armor platoon/squad (4-6) than there are in a conventional infantry (25-30) and yet they occupy the same amount of space on the table top (at least, in game terms). The +1 may reflect the "bigger spread" of the fewer-numbered troopers versus the larger group, increasing the difficulty of hitting. (whether you consider it bigger area to cover with a single attack, or the hex providing suitable hiding places for individual units - the end result's the same in abstract combat).
Mechanized/motorized infantry types (wheeled, tracked, hover, naval, or VTOL) I consider too cumbersome or too clumsy to get the bonus the more nimble battle suit gets, so I wouldn't award them a bonus either (pretty sure they count as Conventional Infantry anyway, at least rationally speaking).