One word - "Time"
Bottom up is fast. Top down could take up to a year or more to design and
implements and usually cost a lot of money with noting to show over that
time. Despite our old beliefs in "Design twice and implement once" as we
stubbornly march down the top down path, the truth is that in most cases the
bottom up can be just as flexible, faster and productive even in phases.
Most business units do not want to wait to see results and would much rather
see an immediate ROI before plunking down the dollars for a full model.
Another reason is, say your company has 10 sales regions, but there may be
up to 20 more in the next five years. At this point it is easier to add the
data mart / region into the larger warehouse. But wait! there was just a
merger and now there are 7 new divisions to add leading to a whole other set
of data marts that would have not been designed in the original model of a
top down approach.
In the end, it all depends on the information, resources and support you
have. Like anything in database design, there is no "one size fits all"
answer.
Hope this helps.