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I remember this D&D 2e book: "The complete book of elves". There are a number of reasons that could cover your question.

If I recall correctly, Elves remain as children for about 70 years (adolescence ending at 100); I could say their minds and bodies develop way slower than humans. They could have great knowledge and dexterity, but might lack the maturity and strength to engage with adults (of any race) or the world’s hazards.

When Elves reach about 101-110, they become adults; it seems this is when they start adventuring (increasing their mortality rate). And so they settle down around age 175 (at this age they become more susceptible to disease); but let’s assume this is the time when they come back home and start pursuing less fleeting matters (such as fame and glory).

Culturally, relationships bear great weight to elves; so they (usually) are picky with their relationships. Any relationship transcending the acquaintance levels require incredible commitment. Elves rarely bond with others, and if they do, it seems it is even rarer having an Elf married. (Marriage’s purpose is reproduction).

After this, let’s say it takes about 30 years for an elf to build a relationship worth their attention. This means Elves are almost 200 years old when they start thinking about family (but what if they die or decide that marriage and children are also very mundane and fleeting?

If you take all of the above into consideration, I think it has to do with their culture.

Elven fertile season is spring (once per year); it basically means they have a lower chance to conceive children than humans (which usually have a fertile season at least once per month). Finally, Elven pregnancy lasts 2 years.