Because against anything except MEQS, three Land Speeders with dual heavy bolters is a far superior anti-infanty choice. Consider: 6 shots per turn with a 48" threat bubble (12" of movement + 36" for the bolter). You'll be wounding on between 2+ and 3+ with no saves apart from cover saves. In every respect, a dual Heavy Bolter is the superior of the Assault Cannon against non-MEQs. Yeah, against MEQ or TEQ, go with the Assault Cannon every time. But against light infantry, dual Heavy Bolter takes the win.why wouldn't I just want either an assault cannon for anti-infantry or a typhoon launcher for range?
Now, there will still be people going 'Meh. There's plenty of anti-infantry that's more damaging'. To them, I say:
THAT'S WHY THESE ARE DEADLY. That attitude right there.
Because dual Heavy Bolter Land Speeders just scream 'Meh', everyone underestimates them, especially with all the other madness on the table. If you play your cards right, make a big show of how 'Oh, I know they're no good, I just play them because <insert lie here>', then while they're dealing with the THSS, suddenly they have 18 shots of death that utterly shred whatever light infantry the enemy are holding objectives with; remember, that 12" movement can get you around the back of cover much quicker than you think... especially if you 'made the mistake' of moving your Speeders out into the 'middle of nowhere' for a turn.
Not to mention: compare the points cost between Assault Cannon/Missiles vs Dual Heavy Bolter. Three dual HB LS's is the equal of a single Dreadnought with an Assault Cannon! That means if they die, who cares? They offer cheap and cheerful firepower, which is something often lacking in the SM army.
TL;DR: the dual Heavy Bolter Land Speeder is an underestimated death machine when taken in squadrons of three. Play them as cheap and disposable light infantry harrassers, and they'll easily win their points back. Use them for enfilade attacks; the enemy will never see it coming, because in this day and age, who uses Land Speeders with Heavy Bolters?