It is because the double bass, essentially, comes from a different family of instruments than the cello, viola and violin.
This is a controversial assertion among music historians, as these things evolved continously, but many scholars do not consider the double-bass to be a part of the violin family at all.
The argument goes like this:
About two centuries before the violin, viola and cello were invented, there was the viola da gamba family: the violone (the double-bass viol), the viola da gamba, the tenor viol, and the treble viol. These instruments came into prominence in the late 1400s, after originating in Spain. These were instruments of six strings, tuned in fourths, like the modern guitar. (and they had frets, too.)
Instruments of the viol or viola da gamba family, from the violone on the left to the treble viol on the right.
The violin, viola and cello were newer inventions that came along almost 200 years after the first appearance of the viola da gamba family. The violin family of instruments came from Italy, and as they were originally conceived, there was no bass instrument in that family. The lowest-pitched instrument of the violin family was the cello.
Violin-family instruments have four strings tuned in fifths. They also have structural differences which give them a much stronger, louder sound than the relatively quiet viola da gamba family instruments. Once the newly-designed violin, viola and cello came on the scene, the viola da gamba family fell out of usage, because they could not compete with the loud, cutting sound of the violin, viola and cello.
Since there was not originally a bass member of the violin, viola and cello family, luthiers adapted the existing design of the violone (the bass viol, six strings tuned in fourths) to make it better able to accompany the new violin-family instruments.
Along the way the violone became longer in scale length, to increase string tension, tone and projection, and it lost its two highest strings. It also lost its frets. This became the modern double-bass. The structure and design of the modern double-bass still has more in common with the earlier viola da gamba than with the cello -- even if, superficially, they look similar.
The Wikipedia article on the history of the cello explains the relationship of the older, lower-pitched Spanish violone (tuned in fourths) to the younger Italian cello (tuned in fifths).