13MHz ISM band vs 433MHz ISM band
Lower frequencies generally means that receiving antennas have larger effective apertures. This means they collect more power but, of course they have to be bigger in order to match the wavelength of the transmission.

A dipole at 433 MHz is going to be about 30 times smaller than one at 14 MHz of course. This is usually the first reason to go for a higher frequency despite the effective aperture limitations.

I would also imagine bandwidth is important. The allowable bandwidth at 14 MHz is very tiny compared to other ISM bands and this usually means that a practical system will only work over a short range due to overlapping interference from other users.

So, in short, antenna practicalities and interference are the main reasons for not choosing the lower frequency.