Because lower electron energy transitions n=1
n=2 or 2 to 3 or 3 to 4 etc are smaller in energy than say, 5
6 or 7
8
and lower energy transitions correspond to lower energy photons which equals shorter wavelengths. So the first few transitions might be very close in energy, meaning their emitted photons will "converge" while the higher energy transitions will have a MUCH larger energy difference and therefore more spread out in the spectrum
Look at the energy of electron levels in atoms and you'll see that it's not a linear relationship between transition and energy required, it's more exponential, and its element specific