Why turn back instead of following the Jurassic Park tour route?

Firstly the map you provided only approximates the layout of Isla Nublar.

Here's what we see on screen...

As you can see they are not the same and there are some essential differences.

We get additional information regarding the layout of the various paddocks when the fences go down.

^{Triceratops Paddock}

^{T-Rex Paddock}

The position of the vehicles is somewhat inconsistent from the action on screen but is close enough.

There is no Dilophosaurus paddock shown on any of the screens but perhaps they are part of a larger paddock.

Is it shorter?

To your main point, it does appear, at first glance, that the distance from the Trike area back to the Visitor Center by continuing on the the anti-clockwise route is marginally shorter than just reversing the tour.

So, why don't they continue that way?

Well, we don't actually know but there may be a couple of reasons.

Firstly, we know that this is an inspection tour where Hammond wants the park seen in it's best condition.

Gennaro, the lawyer says...

Let's get something straight, John. This is not a weekend excursion, this is a serious investigation of the stability of the island.

^Script

Dashing through the last few bits of the tour would be rushed and Hammond might want to them seen at a more leisurely pace.

Secondly, we don't know what that route entails.

The area might be more dangerous in the middle of a storm than just turning around.

It could be shorter but still take longer for other geographic issues like rivers, hills etc.

Thirdly, and more likely....it's a mistake. The movie makers overlooked that the "return" to the Visitor Center was shorter than turning around or just ignored it for dramatic licence.