B2B is an "older" industry, first and foremost. This instant connection between small businesses and customers is pretty new.
I'll give you an example based on the company I work for, ShipMonk. We're a fulfillment company, and the communication between our customers and our system, which allows us to receive and fulfill orders, is enabled almost entirely by API connections. API stands for Application Programming Interface. The basic idea here is that, through an API, both systems are communicating directly. The contrast to this is EDI, which stands for Electronic Data Interchange, which is clunkier and more expensive. I'm not the best person to explain coding or technology, but, if you're interested, you can check out an article that explains the two here.
What I'm trying to get at is that, since B2C is newer, the technology is newer which allows it to keep expanding. It's also considerably easier to manage than B2C, where orders can be notoriously long and complicated. That, coupled with inverse relationship between online and brick-and-mortar stores, allows the B2C sector to keep growing and improving, while B2B lags behind. That's not to say B2B is dead, but I think it will, at least in the technology front, always be a bit behind B2C operations.
I think B2B will keep moving up in the coming years because of the movement of modern retail and ecommerce. B2B business is always a significant part of the economy, which solves plenty of problems for enterprises and I guess that number of B2B companies is much more higher than B2C companies.
Check this article "retail trends 2019" and you will see how B2B business would be developed, due to the movement of modern retail.
And about the biggest challenges for B2B business, I think the anwser is the cost for B2B is much more higher than B2C, and the revenue turnover for B2B is pretty long, some companies earn their income after 6 months delivering their services.