Because Jay-Z is not an artist, he is an imitator. He has to wait for a trend to develop so he can follow it. He observes a formula and then he applies it. He sees who's hot in the game and he latches on to them for success.
Case & Point:
The criminal tales Jay-Z was spittin on Reasonable Doubt wasn't his lane previous to that album. "I Can't Get With That" nor "Da Graveyard" were anything like "D'Evils". It's not until you heard stuff like G Rap's "4, 5, 6", Raekwon's "Only Built 4 Cuban Linx" etc, did you hear him follow suit. Nas did the same, however, What makes him different from so many others was that he was invited into it. He was on G Rap's album, as well as AZ's "Doe Or Die" on "Mo Money, Mo Murder, Mo Homocide" which began The Firm. Jay just jumped in like he was everybody else.
Diddy was dancin' around all dumb with bright videos and flashy gimmicks so Jay-Z made "Sunshine". Ruff Ryders came around and started deadin' that shiny suit shit and Jay-Z released "Money Cash Hoes" ft. DMX. When Hot Boys got play in 99, Jay invited Juvenile to do the hoot on "Snoopy Track". So on and so forth. To answer your question Jay-Z adapts to the times by emulating whatever is popular. He doesn't create anything. So it's not that he's a late bloomer, he just needs something popular to feed off of.