Why?
That's not meant as snarky in the slightest. I'm honestly curious.
Possibly not a direct answer to your question, but here is my answer.
Older editions offered a few different mini-games for the Wizard player that have slowly been removed as conveniences for the class.
What spells do I want in my book?
Spell lists sizes were limited by Intelligence. The maximum size in 1e was 18? spells per spell-level at 18 Intelligence. It was possible to research your own spells above that limit, but the cost in time and cash was quite large weeks to months per spell, many 000’s of gp total expenditure, no guaranteed success or predictable time or cost investment. The strategic game was to determine what spells your optimal book would carry. 2e (and possibly an expansion book in 1e) gave unlimited spells to 19 Intelligence and so that became the holy grail for Wizards to aspire towards (needing 10+ Wishes or a incredibly rare Tome once 18 was reached!).
What spells can I put in my book?
There was a flat percentage chance to learn a spell that peaked at 85% for 18 Intelligence. Rerolls were
possible, but unlikely. What you wanted and what you could use weren’t necessarily the same thing. The strategic game was building the best compromise book you could. 2e watered this chance down by granting a reroll every character level and 3e defanged it entirely by making it moderately easy Spellcraft check.
What spells to carry today?
Since the spells you can access are limited and the day’s plans are known, what spells best suit the situation? Each slot size was independent and you could not store a
Fireball oin your 4th level slot because you thought you'd need another. The tactical game was building the most effective deck for the situation as you understand it based upon your available resources. Is there a way to adjust the plan to take advantage of a specific suite of spells?
What should I do?
Based upon the remaining deck of spells, the tactical situation, and known plans for the day, what should I cast this round if anything?
I think those conveniences helped drive both that the Wizard can replace other classes and the general power level of the Wizard class. Each convenience offered to the class appears unremarkable. Wizards don't need a limit on spellbook size; failing to learn a coveted spell isn't fun, let them learn everything; some spells have greater appeal, let's let a spell fit in any slot equal or greater in size! Combined, those conveniences greatly improve the value of the spell prepared and improve the general utility of spells known to each Wizard.
Allowing a mana point system simply adds another convenience -- and hence more ability -- to a class that already suffers complaints about its overall utility while robbing the class of mini-games I find fun.