The answer is simple and does not imply advance math.
In a normal 4-color there are:
10 (straight flushes per suit)*4(total suits)= 40 straight flush 5-card hand combinations
13 (four of a kind combinations) * (12 the kicker) = 156 5-card poker hand combinations
Therefore 40<156 = it`s harder to hit a straight flush
In a 5-color imaginary deck there would be:
50 straight flush 5-card hand combinations
13 five of a kind 5-card hand combinations
Therefore hitting 5 of a kind it`s gonna be much harder than hitting a straight flush.
P.S. I`m not 100% sure about this way of putting it so pls correct it if you find any flaws