Cost is the major problem, with a couple of technical/deployment issues.
The technical/deployment issues to me are the lack of browser support (that means Edge, Firefox, Safari, etc.), the long and slow migration from USB-A to USB-C, and the missing parts of the mobile puzzle. With the latter I mean U2F support for Bluetooth Low Energy (BLE) and NFC on (at least) smartphones.
Ideally, you could visit some secured website on your smartphone, choose to authenticate with Fido U2F, tap your U2F key to the phone, and authenticate with it using BLE or NFC. The same key can be used on a laptop or desktop computer as well using USB.
Those devices will exist (or already exist perhaps), but they will cost a lot more than the plain USB-A U2F keys available now for roughly $15.
To drive adoption, ideally banks would get on board and go for U2F. That way a lot of people would come in contact with the technology, driving adoption and prompting users to use the key for other services as well (for the bank this provides a nice branding opportunity!).
Unfortunately, banks tend to favour private solutions based on TOTP/HOTP in a lot of countries. That means that in, for example, my native country of the Netherlands you will get a small battery powered calculator-like device from your bank that generates the challenge-response verification codes needed to authorize transactions. Each bank has its own solution that only works with them, and each will send you their private branded TOTP-in-a-box device.
Add to this governments that are attempting to introduce electronic ID-cards containing NFC-chips for public authentication with government and commercial entities alike, and you can see why in a lot of countries the only candidates for U2F are global services like GitHub and Dropbox. That reduces the amount of potential U2F users to what are essentially power users.
Just a heads up: on the US Amazon site Feitian has a USB-A + NFC token for $16 (and there's a one-per-customer coupon on the amazon product page to knock it down to $10).
Feitian also have a BLE + NFC + USB token for $24 (with a coupon to buy it for $16), but that requires charging a battery, is less rugged, and the USB requires a cable to connect to it.
It's not as cheap as USB-only (there used to be a $6 USB token sold), but NFC support doesn't have to cost much more (especially as the secure element chips they're built around all move towards having NFC support as a baseline anyway).
Also there seem to be a handful of Java Card implementations of U2F on github already (one of them is even sold as a Fidesmo app, if you want to pay for easy installation), so an NFC-only U2F token could presumably be had for as cheap as any javacard-compatible NFC smart card, and then just registered as a second token.
I don't think it's enough to help push U2F forward by itself, but I think if webauthn can get solid cross-browser support for U2F implemented, price won't continue to be a big problem. Having just read up on webauthn, and seeing how many browsers already have test implementations shipping, I'm pretty optimistic U2F is going to be seeing a lot more interest soon.
The technical/deployment issues to me are the lack of browser support (that means Edge, Firefox, Safari, etc.), the long and slow migration from USB-A to USB-C, and the missing parts of the mobile puzzle. With the latter I mean U2F support for Bluetooth Low Energy (BLE) and NFC on (at least) smartphones.
Ideally, you could visit some secured website on your smartphone, choose to authenticate with Fido U2F, tap your U2F key to the phone, and authenticate with it using BLE or NFC. The same key can be used on a laptop or desktop computer as well using USB.
Those devices will exist (or already exist perhaps), but they will cost a lot more than the plain USB-A U2F keys available now for roughly $15.
To drive adoption, ideally banks would get on board and go for U2F. That way a lot of people would come in contact with the technology, driving adoption and prompting users to use the key for other services as well (for the bank this provides a nice branding opportunity!).
Unfortunately, banks tend to favour private solutions based on TOTP/HOTP in a lot of countries. That means that in, for example, my native country of the Netherlands you will get a small battery powered calculator-like device from your bank that generates the challenge-response verification codes needed to authorize transactions. Each bank has its own solution that only works with them, and each will send you their private branded TOTP-in-a-box device.
Add to this governments that are attempting to introduce electronic ID-cards containing NFC-chips for public authentication with government and commercial entities alike, and you can see why in a lot of countries the only candidates for U2F are global services like GitHub and Dropbox. That reduces the amount of potential U2F users to what are essentially power users.