"Open D" (major) and "Open E" tunings are really the same, relatively speaking, and the old bluesmen recognized that fact by calling it "Vastapol tuning" regardless of absolute pitch (after a well-known tune played exclusively in that tuning).

To retune from standard to open-D, three strings are lowered in pitch (loosened); to change from standard turing to open-E, the other three strings have to be raised (tightened).

Acoustic players generally prefer the open-D version of Vastapol, preumably because tuning up to open-E often results in prematurely broken strings.

The extra-light strings customarily used on electric guitars seem better able to stretch without snapping, so electric players are somewhat more prone to tune up to open-E.

Another reason to opt for the open-E alternative is that the key of E is far preferable to D when playing blues on the guitar in standard tuning. For a slide-blues player in a band (or even a duo) with one or more standard-tuned guitars, use of open-E tuning is pretty much universal.

As Jed observes, you can tune to open-D and capo up two frets to play in E, but simply tuning to open-E in the first place is easier and, for light-strung electric guitars, not a problem. I'm almost certain that Duane Allman used open-E very regularly.