I had a phone with a terrible web browser, a tiny screen, and ridiculous charges for bandwidth before that. MapQuest likely had a terrible mobile site for it to talk to. I think it's likely that I theoretically had maps on my phone 8, 9, or even 10 years ago.

In practice though, I didn't have them until June 2007.

Maps worked fine on the Palm. Not as nice as today's maps, but they were sufficient. Also, Sprint introduced a reasonably priced uncapped plan around 2002 or 2003 or so, coinciding with the introduction of one of the Treo models, so there wasn't really a data-charges problem. In fact, the data plans got worse for a period later in the decade. There was a time in the late-2000s when Sprint re-introduced per-MB bandwidth charges, except that Palm phones were grandfathered in on the old plan, so you got unlimited data on Palm, but not on anything else.

It's true that uptake has increased over the last decade, though. Partly due to product improvements, but I think largely due to cultural changes.