That is a slightly random and very non-specific question which makes the obvious answer to it "Because they want to" but I suspect that's not what you were looking for.
The fact that you have started a number of threads through the night that had a distinctly christian theme (and because of some other posts you've made) would suggest to me that you don't like the idea of halloween or Samhain as it was originally known because of it's pagan roots, and that your question has more to do with that, but that is just a guess on my part and past experience tells me that guessing what other people actually mean or are trying to get at can often go very badly.
What I am trying to say is the there are many answers to your OP question ranging from personal to historical and probably beyond so it might be more productive if you were a little bit more specific about what you were asking - this is an observation not a criticism.
These days I don't really do the halloween thing but, personally I'm not keen on the whole pumpkin thing, I'm a Celt so my love of Samhain is cultural and there were no big orange pumpkins in my heritage but I absolutely get that they are much safer to carve than root vegetables.
The thing that most bothers me about this new pumpkin tradition is that it is very environmentally harmful, buying pumpkins to carve and discard produces huge amounts of methane as they decompose. This is pretty much the complete opposite of what Samhain is supposed to stand for.
