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Several early programming systems did use custom character sets (custom type bars) as well as typewriter-oriented notations like overstrikes, half-line motions for superscripts (powers) and subscripts (indexes), and colour (red half of the ribbon for comments)¹. Except for APL, which had IBM behind it, these tended not to escape their original research environments.

Relatively large-scale production required I/O devices to share a common character set. These tended to be based on pre-computer character sets (teletypes, punched cards) and were limited in size both by the I/O devices and by storage requirements (don't forget how small and expensive memories were). A lot of work went into choosing character sets.²³⁴

1963 ASCII had an up arrow ‘↑’ dropped in 1967 in favour of ^ (because furriners wanted accents), and a left arrow ‘←’ dropped in favour of ‘_’.⁵⁶

¹Jean Sammet, Programming Languages: History and Fundamentals 978-0137299881
 ²Charles E. MacKenzie, Coded Character Sets: History and Development 978-0201144604
 ³http://www.trailing-edge.com/~bobbemer/SURVEY.HTM ⁴R W Bemer, Design of an improved transmission/data processing code http://dx.doi.org/10.1145/366532.366538
 ⁵American Standard Code for Information Interchange http://www.wps.com/projects/codes/X3.4-1963/index.html
 ⁶Revised U.S.A. Standard Code for Information Interchange http://www.wps.com/J/codes/Revised-ASCII/index.html