Well, a crib is really for infants, toddler beds are for toddlers, and she's older than either of those now that she's a preschool age kid.
Baby bed mattresses have a weight limit then whomever is sleeping on them can feel ever spring, they have about zero support, and it's flat out uncomfortable.
She should be in a regular bed by now. It's a wonder she isn't climbing out and wandering around.
I'd say she is waking up because she's had enough sleep. Perhaps you can adjust her sleep schedule so she goes to bed later and when her hours of needed sleep are up she'll wake up a little later. Kids only need so many hours of sleep per day. If she's going to bed by 8 pm and waking up by 6am then maybe if she goes to sleep at 9pm she'll sleep from 9pm til 7am. They're too little to sleep more than 10 hours at night, their bodies are screaming for food by that time.
It should also be dark in her room. If she's waking up for the day then that's on you. You are putting her down too early for bed. If she's a fussy sleeper all of a sudden then she is probably about to have a growth spurt and her body requires food in the middle of the night. Feed her something and then put her back down. If she's been a good sleeper and is now having these issues then look at the environment. Is she hearing something at that time? Is she done sleeping for the night? Did the neighbor leave for work at that time? Does the heat and air kick on the same time each morning to start a new cycle? We have ours set to come on and bring the house temp up around 7am. It kicks on and goes for a while to warm it up.
There are many things that can trigger her to wake up. But if she's simply done sleeping the hours she needs for her body there isn't anything you can do but get up with her. Or put her to bed later and get her in a different bed.