Well, i will make no claim to know why VINTAGE cues were heavier, as i am not that old, but i will give an opinion as to why some people might prefer them over a ligher cue.
First let me say, that i have a long pendulum style stroke (on par with any of the longer bridging filipinos)
In the past, i was afforded the opportunity to hit balls with Efrens old cues, and some of Parica's old cues. (maybe about 12 years ago)
To my complete amazement, these things (especially efrens) weighed a TON.
I could not fathom how they could possibly make balls with these things, and have some semblance of cueball control.
To me, they felt like playing with LOGS.
Then, in the numerous discussions that resulted, it was told to me that a heavier cue makes for a straighter stroke. (IF you have a pendulum style stroke where you just basically drop your arm)
The theory behind it is, that using a lighter cue, you actually have to do more muscle work when delivering a stroke, and that if your stroke is off a little, or that if your arm is tired for instance, that it would be very easy to falter, and have a hitch or some other problem that would throw your stroke off by a little, occur
With a heavier cue, you don't need to use as much muscle to force the cue. You basically just let your arm drop, and assist the cue in the motion, but you never really have to force it till a power shot comes up.
The heavier cue does most of the work FOR you, and by the mere weight of the cue, PULLS your arm as it's swinging like a pendulum, and keeps the stroke straight. (this is assuming that you have one of these strokes to begin with) The heavier the cue, the less your arm will pull it out of the strokes path.
Now i know your gonna have people that say 1 or 2 ounces can't make that much of a difference in a stroke being straight, but at the same time, i'm pretty sure that the majority of serious pool players are going to know instantaneously if a cue is to heavy or to light and if the balance is what they are accustomed to or not, based on only 2 or three shots attempted with it.
SO...i'm willing to bet that a couple of ounces CAN make a difference, even if it is just a slight one.
i know that most of my cues are about 21.5 ounces, with 1 being 20.75. and i know that i never wonder if my stroke is straight when using them. I wonder when i'm using a cue that feels like a feather, but never with one of the heavy ones.
Just some food for thought.