It's also that all the stages were turned into empty squares.
Well let's just drop some numbers regarding stages...
DoA3
has 14 stages overall
5+1 are "empty squares" (5 tag arenas and Tao)
4 stages are multi-tiered
DoA4
has 14 stages
3+1 "empty squares" (3 tag + biolab core..)
7 stages are multi-tiered
*note: not counting "Kyoto in Bloom Night version" and Dojo. DOATEC great hall counts as single tiered, since it only loops back into itself
DoA5:U(LR)
has 24(26) Stages
8 "empty squares"
9(11) multi tiered stages
*note: not counting Haunted Lorelei and Dojo, did not count the lowest lvl of a multi-tiered stage as an extra stage
**an "empty square" has no meaningful stage interaction besides walls (well and maybe a slight slope)
So the ratio is about the same as DoA3, and DoA5 wins everywhere when it comes to absolute numbers.
If you only look at pure stage interaction a lot of the stages are not bad imo... there is quite a lot of different stuff to interact with in many of them. But I guess art direction is really somewhat poor... e.g. DoA4 stages had more attention to detail, or at the very least more stuff in them.
I think Hotzone is a good example, there are a lot of different dangerzones (RPG, normal walls, partition PB Zone, Tank, that explosive door thingy, PB Dangerzone, could have made those guys with the shield into a "bouncy wall"- like those balls in the circus stage), but the stage just looks bland...