Wouldn't it be easier for the end users if all microphones were USB? Then why are most of them using XLR?
Simple answer: because microphones are analog and USB is digital.
The only way you can make a "USB microphone" is to build an analog-to-digital converter into it. Of course you can buy the electronic components to do that with garbage sound quality for a few dollars, and for some applications (e.g. headsets with microphones) that is fine, because audio quality doesn't matter. For some Bluetooth wireless audio systems, the frequency bandwidth is no better than a 70-year-old telephone.
You can't power a standard design of mike that uses the standard 48 volt "phantom power" over a 5 volt USB cable either, except by adding yet more interface components.
Not to mention the fact that the maximum length of a USB cable is 5 meters which is completely inadequate for many recording situations. Professional audio recording often uses 100 meter cables in a large venue.
If you want professional results you use the right tools for the job, and a USB microphone is unlikely to be that tool.
A USB microphone is essentially a microphone, preamp, and audio interface in one. They're convenient when you're recording or streaming with a single microphone that can be placed within a short distance of the computer (the specification for USB has a maximum cable length of 5 meters). But they become almost unusable outside of that situation.
A live sound setup usually involves multiple microphones, perhaps run a very long way, and then plugged into a mixer, with signal routed to potentially many places. USB microphones have no use here.
A professional recording setup also usually involves multiple microphones. It's not impossible to use multiple USB microphones simultaneously, but it's a major headache, not the least of which is the additional strain on the computer to connect to multiple audio devices. And then there's also the issue of the maximum USB cable length.
No, for a few reasons.
1) USB isn't as transient and XLR isn't. What happens each time USB is upgraded?... buy new mics? The only major change to XLR was the introduction of 1986 phantom power in 1986 - gave mikes power without batteries ... the power supplied along cable.)
2) High-end mics are expensive, and sometimes older (pre-digital), better mics are sought out. The studio that can spend $9k on a paired set of Neumann U-87's won't have a cost issue with a $1000 audio interface.
Capitol Studios in Hollywood still has their #1 microphone, Neumann U47, $10k value, is still the featured microphone available to use.
3) USB has no fidelity advantages. (Mics could just as well be Ethernet or Bluetooth or USB Wireless.)
It might be more convenient for the casual computer user who just wants to record a podcast with a single mic. And that market is catered for. But that isn't what 'all microphones' are used for.
Also, latency. The audio interface in a USB mic is a pretty basic one. It won't have a low-latency ASIO driver like your Focusrite has.
Microphones with digital rather than analog interfaces work with a certain sampling rate and latency. When working with more than a single signal source, sampling rates and latencies have to be synchronized. D/A converters with reliable sample synchronization are way outside of the price league of what you can sensibly build into a microphone, and USB does not provide mechanisms for such synchronization: it would have to be done externally.
A typical drummer alone warrants using at least 4 microphones of at least 2 different types.