Hi Rick, I'm not sure where you live, but I have a few thoughts on this one.
Jeans are very popular for men and women in the UK for casual wear, but I guess long tops, dresses and leggings are also very popular with women as this is pretty comfy get-up.
I think there are a few reasons why women might not wear dresses as much as they used to and these are my thoughts.
Dresses can be kind of annoying if you're not wearing leggings with them. If they are knee length or above you have to be constantly mindful of how you are sitting etc. Otherwise you are going to be flashing too much leg and people might see your knickers

I am a grown woman and I know men will notice this sort of thing and most of them will even be looking out for it, who wants to have to think about that all the time?
Also, dresses invariably show more leg than trousers do. At work gender and a womans legs should not even be a factor. It is all about your capability to do a job, so trousers are probably more practical (again, it's the sitting thing and potentially giving people an eye-full).
Dresses are also a bit flouncy and some of them flap around, ride up, get caught on things more easily etc, so again, trousers are often more practical. Then there is the issue of your legs getting cold and that even happens in tights.
Last of all you have to constantly have legs hair free if you wear dresses. I speak as a lady who waxes my legs. It's great when you've just had it done, but you have to let the hairs grow long enough to wax them again! So, you will probably be wearing trousers when it's coming up to waxing time!
I do like to wear dresses though, but they are not always the most practical attire and I think that's the key to why women have chosen to wear trousers more often.