In Ukulele TAB, why is the 4th string represented by the bottom line and not the top line?

Let’s suppose we have a Low G strung Ukulele. Low G strung ukuleles have 4 strings: 

String 1 (A), String 2 (E) , String 3 (C) , and String 4 (G).  

Low G strung ukuleles and High G Ukuleles both are tuned to Standard C6 tuning. 

Meaning we tune the strings to the notes of a C6 chord when strummed.  G4 C3 E2 A1.

Low G ukuleles are tuned so the strings are in Ascending pitches,  Low to High (G4, C3, A2, E1) . 

High G ukuleles are tuned in re-entrant tuning. Meaning the 4^th string is Higher than the 3^rd string.

This is similar to a 5-string banjo, which has a high droning 5th string followed by strings 4, 3, 2 , that start at a 

pitch lower than the 5th string and “re-enter” into Ascending pitches.

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When we read Ukulele TAB we relate it to Low G Tuning because the strings are in an Ascending Order of pitches.

Musical Scales are written from the lowest note (The Root Note) and ascend to the highest note ( The Octave) .

When we read TAB or TABLATURE, we imagine holding our ukulele flat on our lap,
looking down at it and reading it like a book from left to right. 

When viewed this, the 1^st (A  string) appears on the top and the

Lowest or 4^th string (G) appears to be on the bottom. 

TAB is a simple form of musical notation. 

We often see ukulele music that has both Musical Notes and TAB.

They are always stacked with the 4-line TAB STAFF below, and the 5-line Music Note Staff above. 

In short:

To make Musical Notation (Note Reading) and it’s corresponding TAB easier to read together, 

the lowest line of the TAB STAFF, the 4^th line, is always on the bottom.

This matches the way musical notation is written. The lowest pitch is the note on the lowest line.