Because Karr talks like he did it and he would be considered a flight risk (he's already skipped out in California). That's probable cause (not enough to convict but enough to hold somebody over). Boulder finally had a location on him so they swooped in and took him while they could knowing he might disappear again if they didn't.
FWIW, I don't have a problem with apprehending Karr (although one has to factor the cost/reward aspect here given who would have to pay for all of this). What I have a problem with is the media hyperventilating over this before hardly any evidence linking him to the crime is presented. We already know his entire family tree, all the family's dirty laundry, etc. yet NOBODY can even place this guy inside the state of Colorado in 1996.
I'm still waiting for some evidence of that, even hearsay evidence. Nope. Zilch. Zero. Nada. Unless the guy is Peter Pan, he needed some means to get to Boulder and I've yet to hear any evidence to that regard. As flimsy as the Karr family defense is that he was with them, it's still stronger than anything the prosecution has come up with to say he was in Boulder (except for Karr's own statement which I disregard without any corroboration).
As for the media hyperventilation, I think that the timing of the arrest was designed to take advantage of the media. They waited until the Israel-Hezbollah cease fire stopped news from coming from the Mideast, and a vacuum needed to be filled. The whole shirt-and-tie, champagne flight was designed to elicit maximum hype. If indeed John Ramsey is involved, he must be frantic right now. Hasn't he called for the public not to "rush to judgment"? If my kid's killer had been found, I'd be angling for a seat at the execution.