A bit of because that's what they use now. And a bit of because you can pretension them to what ever you would like and reset the tension removing any play that develops.
Uhmmmm... I don't think that's absolutely true. you may still need shims, depending.
The tapered rollers are also larger to take big loads with less chance of failure.
Larger rollers have a downside though - with a larger diameter, you're rolling back and forth over the same space more, thus uneven wear. With the smaller needles, you have more overlap thus more even wear. And if you have smaller needles, you're spreading the load over more rollers.
They also handle loads in three dimensions instead of the needles two.
I think the word you're looking for is "vector", not "dimension". And, in this case, they're actually worse, since three vectors are involved. Up, down, and sideways. With needles, you're only dealing with up and down - the spacing/end is only for locating, and not involved in actually holding the bearing in place, and overcoming forces trying to squeeze the bearing out the end.
In my limited sample of one bike done so far and by the manual, the side play is to be adjusted with shims in the stock set up. the bike I rebushed required an extra spacer (washer) to reduce the side play without putting what I felt would have been excessive bending stress and misalignment of the frame spigots to the axle by the amount of movement that would have been required to take up the slop otherwise.
Oh no. I entirely agree that general maintenance is very much easier with tapered vs needles. Which is probably a big reason in going to them in modern bikes. But a lot of design goes into "how much quicker/easier is it to assemble on the line/work on in the shop" vs "what is the *best* design".