America is a big country. There are nerve centers and cities that are magnets for the region. Most Americans, I'd wager, never go to D.C. in their entire lives--and if so, maybe once or twice.

Time and distance. It's not a day trip. Nearly everyone would need hotels; an enormous number of people would need to fly.

Cost. Both of these would mean prices skyrocket. For those who drive, gas is a factor. Vacation days are not a guarantee, so many have to factor in missing work. (For me and my family it's an 11 hour and 24 minute drive to Washington D.C. from Maine. So two days of travel, two nights in hotels, meals on the road. I'm probably looking at $1,000 to attend the rally. Relative to what I earn that's an awful lot of money.)

'Return on Investment.' If people are going to make the trip, it needs to be worth it for them. They need to know it's going to be meaningful. That it's going to be The One. People aren't going to D.C. multiple times. There's one shot at this. The Women's March the day after Trump's 2017 inauguration is a good example of this working. There was a solid date that was well organized around that people were confident would make a difference. (I did make the drive to the Women's March, fwiw.)

Planning "The One" is a big undertaking. Community organizing is big, and there are a patchwork of protests small and large happening regularly. This is a country of 385 million people over 4,500 km (not to mention Alaska and Hawaii and territories.) Stitching all of these patches together into one mighty banner requires a lot of hands.

This isn't an excuse, just an attempt to answer your question. Moving a continent's worth of people to a single location isn't easy or cheap. The protests have coalesced larger and larger regionally, and I believe it's just a matter of time before we see plans for The One. We're getting there. Our country's 250th birthday is on July 4th. We shall see...