Because the regular ice cream has fat, and it is not light.

Ice cream gets its delicious texture and properties from 3 things: small pockets of air, ice crystals, and fat globules. If you have ever made ice cream, you will know that churning it is very important. This is to evenly distribute air into the cream mixture (otherwise you would have frozen milk, which isn't nearly as tasty). If you churned the cream mixture without cooling, all you would get is butter and buttermilk. As you churn the fats create a foam (air pockets), which is then frozen as an emulsion (two things which don't normally mix) with the other ingredients.

The amount of fat in the ice cream does seem to play a significant role in determining its melt rate- but so do other factors, such as salt, the amount of air (density), and 'stabilizers'. I would check the ingredients of the low fat and light ice cream, they may have ingredients such as polysorbate 80, which is an emulsifier, reduces ice crystal size (makes it smoother), and increase melting time. I have a feeling that the chemistry required to make 'ice cream' without using fat (no cream!), is the reason they take longer to melt. Other factors include the 'hardness' or density (amount of air).

[Interestingly it seems from reading online that more dense ice cream actually melts faster. This might be a good experiment to try. Take two scopes of ice cream, and let one scoop melt completely, then refreeze it. Then compare the melt rate of the refrozen scoop to its twin.]

You would be surprised at the amount of serious scientific studies into all things ice cream. I have linked to one below.

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