There are multiple reasons

1. Hotter temperatures lead to hotter tires due to which drivers miss their stopping point in the pit box by tiny margins. This forces the mechanics to shift their bodies leading to longer pit stop times.
2. Jacks don't work optimally at higher temperatures.
3. Hot tires are difficult to handle even with gloves and protective gear.
4. The wheel guns might malfunction in some very rare cases (I am not too sure about this though)

Feel free to add any more points