Because it's the standard and 3-14 doen't offer any advantages. Once upon a time, what we see as routine automation and remote monitoring was bleeding edge and the pervue of instrumentation and telemetry companies. There were fewer companies in it and the vendors were competing on the same jobs so it was to their advantage to work on the same wire. Not long afterward, someone who will go nameless figured out that by enhancing the standard to carry more information, they could simultaniously claim cost savings and shut out competition from the non-enhanced vendors. The rest, as they say is history, although in this unusual case the others didn't respond by each adopting a different and arbitrary range as the installed base was such that it was still in their interest to stick with the earlier standard. The market grew and the vendors decided that it would support 50 zillion totally incompatible "standards" and that's where we are today. To my (limited) knowledge this and binary expression are the only universal standards in effect in the industry. The various RS serial standards would be contenders (again from outside the industry) except the norm is outside the standard and various vendors do really perverted things through ignorance or for competitive purposes. In the interest of charity, I attribute these to the latter, although simetimes I wonder. The situation does provide for some with the knowledge, skills and hardware acumen to eke out a living getting things to work as the customers expect and as they should have in the first place. And it's much more interesting than simply operating end user shrinkwrap software like everyone else.
Because it is an IEC/ANSI/ISA Standard. If you want to know why, check the archives. We just went over this last month.
The basic reason is that it provides enough power to run many instruments, yet not enough power to make intrinsic safety impossible, and allows a failure alarm at 0 mA; while easily converting to 1-5 VDC using a simple dropping resistor.
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Whether it's 4-20 mA, 1-5 Volts, or 10-50 mA (old Foxboro standard) you'll notice that the span is a multiple of 4. This is an easy base to work with when scaling signals. 3-14 gives a span of 11 - a prime number and not so easy to work with.