Because they all sell quite well.
Certainly, but I think it's due to having a reputable name like UA at an affordable price. The single digit Volts are very similar to the 76-Volts but inferior. It would always make more sense to buy the 76 versions, and there are several ways to improve them:
- Enhancing the headphone amp. It sounds amazing on my Beyerdynamic DT 770 Pro 250 Ohm headphones, but more output level would be highly appreciated.
- The Audient ID14 is available for around €211 and comes with monitoring software that is just as good as the Apollo software, while the Volt software can't keep up with them. It also has an optical input.
- Upgrading from USB 2.0 to (at least) 3.0. Without an external power supply, you are completely dependent on the 500 mA of USB 2.0. This is not enough. 900 mA from USB 3.0 would be much better. Also, USB 3.0 is full duplex and supports data traffic up to 4.8 Gbit/s. That would even make it possible to connect an external SSD drive to the interface.
As I see it, mainly beginners and novices opt for the single digit volt interfaces. After 6 to 12 months, they feel they have "outgrown" them and want a new interface. At that point, they usually plan with a budget of €500 and tend to lean towards Audient, Motu, or Presonus. Alternatively, they may opt for used interfaces from RME. At least, that's the general sentiment found in German forums. Therefore, I believe that the strategy with single digit volts and 76 volts may not necessarily be sustainable.
For my needs in home recording, my 476 Volt is completely sufficient. However, when it comes to my band, we use an Apollo x6. Although the Volt 476 is an excellent interface, there is plenty of room for improvement. Furthermore, I believe that a specialized version for guitarists/bassists would also be well-received.That is just my opinion. You probably see it very differently.