It's hard to answer as traditionally using the same font wouldn't have been an option at all, you simply need too many symbols. Consider the small document

\tracingonline2
\showboxdepth100
\showboxbreadth100
\scrollmode
\setbox0\vbox{
one {\it two}
$$\alpha + \sum_0^\infty x^2$$
}
\showbox0
\expandafter\show\the\tenrm
\expandafter\show\the\tenit
\expandafter\show\the\teni
\expandafter\show\the\sevensy
\expandafter\show\the\tenex
\box0
\bye
That uses 5 fonts as the output shows:
> \tenrm=select font cmr10.
> \tenit=select font cmti10.
> \teni=select font cmmi10.
> \sevensy=select font cmsy7.
> \tenex=select font cmex10.
the upright roman text font, the italic font, the math italic font a 7pt symbol font and the 10pt large symbol extension font.
Classic TeX simply uses too many characters to fit in one 8bit font.
Perhaps you were mainly just asking why the math italic font is different to the text italic? There mainly the difference relates to teh fact that in text adjacent letters form a word so the letters have small sidebearings so they are close together and some combinations are merged with ligatures or brought closer with kerns. Math italic conversely adjacent letters are typically an implied product of separate variables and the design tries to make them not look like a word: wide sidebearings and typically no ligatures.
With a modern Unicode font there are of course possibilities to put many glyphs into the same font but that is just an implementation issue, you would still want different glyphs, perhaps using an italic font with U+0061 for an a and U+1D44E for a math italic a, so even though they are two a in the same font, they are different glyphs, just packaged into one big font rather than lots of small ones.
OT1
encoding which is the default for text. You can useT1
encoded fonts in place of these if you useT1
for text. However, you are still going to need everything else from dedicated maths fonts (e.g. encodedOML
etc.) and maybe dedicated text symbol fonts (TS1
).OML
encoded font to typeset text or aT1
encoded font (alone) to typeset maths. So from the end-user perspective, it is reasonable because it is the only way. From the designer perspective, it is reasonable because that was the only way to set the system up given the constraints which applied at the time TeX was written.