With JQ undercutting it on capital city routes and QF adding 45% capacity on regional routes meant even if Rex had a competent board things were stacked against. Slots were just one more thing as being denied slots when the flight times were more profitable.
Flights at peak times only tend to be more profitable because they carry business travellers who typically command higher yields. Rex was mainly targeting leisure travellers and had only just rolled out a relatively abysmal and limited frequent flyer program with very little appeal to the types of travellers that make flights in those time slots higher yielding. Flights also tend to be higher yielding when they are closer to being full, as passengers must purchase higher-priced fares. Rex's jet flights were rarely full because it just didn't have the overall product offering of a business-compatible airline. (Had they had even more flights at peak times, each flight would only have been emptier meaning higher per-flight costs and reducing yield further, thus, increasing their losses even further.)
Heck, they only recently began accepting credit card for inflight purchases - before then it was cash and coin only, even on jet flights. I wouldn't have carried AUD for a good 10 years before Rex entered the jet market, so while there's always a need to fend off competition, Rex waited too long to make a serious effort of it.
To a previous question, yeah, Rex also based its entire jet investment strategy around Virgin Australia not emerging from administration, the goal being to fill VA's place (buy VA's planes, take VA's workforce, operate many of VA's routes). They called it Project Mother... but they didn't stick to their own timelines. The frequent flyer program was supposed to launch a couple of years before it did. There was supposed to be a brand new lounge built in Sydney. Melbourne's lounge was supposed to have been expanded out into the terminal. A new lounge was to be built in Adelaide. A new lounge was to be built in Canberra after using the then-abandoned international departures area as a temporary lounge during COVID. All of that was severely delayed or in the case of lounges, didn't happen.
Having no solid lounge network, no solid frequent flyer program, no true status benefits until the very end where they launched in a limited way, a limited jet network, no actual partnerships (a baggage through-check with Delta was about as serious as things got - heck, even Qantas has that with Delta, and United, and...), no mobile app, fees tacked on of up to ~4% just to buy a ticket online (which were avoided if you took a briefcase of cash to an airport ticket counter, because apparently that's more efficient...), no ways to earn points with on-ground partners... the list goes on. Rex can cry all it likes about having competition in the market, but even as a newer jet market entrant, they had every chance to design, roll out and market their product in a way that would attract more travellers, including those higher-yielding business travellers. They failed in just about every respect, and on that front, only have themselves to blame.
