An opening act at a large concert typically plays for a fraction of the headliner's time. The length is set by the mechanics of running a venue rather than by any judgement of quality.
The curfew works backwards from the end
Venues operate under noise and licensing conditions that fix a hard finishing time. Everything in the evening is scheduled backwards from that point rather than forwards from the doors opening.
The headliner's set length is fixed first, because it is what the audience bought. Whatever time remains is divided between the support and the changeover.
Overrunning a curfew carries financial penalties and threatens the licence, so the margin is protected carefully. The support slot is the flexible element, and it absorbs any compression.
Changeovers take longer than the music
A headline production occupies the stage almost entirely, with the support squeezed onto a narrow strip at the front or performing in front of a closed curtain.
Even so, microphones must be reset, monitor mixes changed and instruments swapped. A crew can do this quickly, but a full changeover still consumes a substantial block of time.
Limiting the support's equipment keeps that block short. A band arriving with an elaborate setup would need a longer changeover, which comes directly out of somebody's playing time.
Bar revenue depends on the timing
Venues make a large share of their income from bars and merchandise, and both sell most heavily before the headliner takes the stage and during the interval.
An opening set that is too short leaves the room empty and the bar idle. One that is too long keeps the audience in front of the stage rather than queueing.
The conventional length reflects that balance, holding the crowd in the building while leaving a clear window in which most of the trading happens.
The slot is frequently bought
On large tours the support position is often a commercial arrangement, with a label or management paying a buy-on fee to place an artist in front of the audience.
The payment reflects the exposure rather than the performance, which is why a relatively unknown act can appear on a major tour without an obvious musical connection.
Where no fee is involved, the support may be chosen by the headliner personally, and those pairings tend to produce longer sets and better sound because the headliner asks for it.
Sound is deliberately constrained
The mix for the support is usually set conservatively, partly because there is no time for a full soundcheck and partly to leave headroom for the main act.
A support act that sounded as loud and full as the headliner would flatten the evening's dynamic, and the touring engineer controls the system rather than the venue.
Acts that tour repeatedly learn to build a set that works under these conditions: short songs, minimal setup and a mix that survives a room that is still filling.