The show was cancelled
"Spiderman Unlimited" aired in 1999, when fox had the rights to the show.
Sadly it ended on a tragic cliffhanger and Fox themselves eventually commented and had stated they cancelled this show in 2001 due to poor viewership and because of the popular shows like Pokemon and Digimon overshadowing this show.
I can't find official wording but I found other credible websites that have put their word in.
According to Nerdist
Batman Beyond premiered around this time, and with its rich backstory from Batman: The Animated Series, combined with new original characters, it was the superior show. Spider-Man Unlimited tried to cash in on that futuristic, techno trend, but the execution was hokey and forced. That’s why the series ended on a cliffhanger.
According to Marvel Animated Universe Wiki
The show was vastly overshadowed by the anime Pokémon, which began airing at the same time and garnered far higher ratings. The series had been doing well on its own, pulling in mid-to-high 3.0 ratings, airing against Pokemon. Despite only airing three episodes, the series was put on a two-month hiatus following "Where Evil Nests", where one month later it was announced that the series was being cancelled.
Rick Ungar, who had then recently been named president of the newly formed Marvel Character Group making him responsible for Marvel's television animation projects, stated that Fox needed to devote more time to shows that could compete with Pokemon.
Fox Kids had been in fourth place the previous year behind Kids' WB, Nickelodeon, and ABC but had managed to move back up to third. Fox kept shuffling the shows around the times, putting The Avengers: United They Stand in this show's slot, with the programming changing from week to week with only Sherlock Holmes in the 22nd Century, Digimon, and Beast Machines: Transformers maintained their timelots. Without a steady timeslot, audiences had a difficult time watching it week-to-week.
Between Marvel's bankruptcy problems, the comparatively low ratings, and fact that it fulfilled the thirteen-episode contract need the series was picked up for further episodes. Although the season one ended on a cliffhanger and several scripts were written for a second, no more came from the series.