How come Ubuntu 18.04 LTS has a 32bit iso installer?

The link provided is for Ubuntu Budgie, an official community-maintained 'flavour' which uses the Budgie desktop environment, not for the "standard" Ubuntu image (which uses the GNOME desktop environment).

Ubuntu's developers decided to drop 32-bit image for vanilla Ubuntu (in fact it's already dropped, there is no Ubuntu 17.10 32-bit image), but (at least some of the) other flavours will continue supporting 32-bit systems for now.

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If you want to get a basic image for Ubuntu 18.04, see the answer by @sudodus.

If you want a 'clean and basic' iso file of Ubuntu Bionic to be released as 18.04 LTS, you can download a Ubuntu mini.iso alias Netboot iso file. There are versions for 32-bit as well as 64-bit kernels.

During the development phase (until April 2018) you will find the Bionic mini.iso files

• via the ISO testing tracker
  • via 'Bionic Daily'
    • for 'Bionic Netboot i386' (32-bit) and Netboot amd64 (64-bit).
      • 'Link to the download information'

You will find more details and the released mini.iso files via this link,

mini.iso, minimal install, netboot iso

You can use the mini.iso to install standard Ubuntu as well as the community flavours of Ubuntu. You can also create an Ubuntu Server or a custom system with only some manually selected program packages.

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But Ubuntu Desktop (standard Ubuntu) and Ubuntu Server 18.04 LTS will only be released with 64-bit iso files (for standard PC computers) as described in the answer by @pomsky.

For your information Ubuntu MATE has 32-bit installer.

You can visit official website https://ubuntu-mate.org/download/ and choose 32-bit version.

Currently they provide both 16.04.5 LTS and 18.04.1 LTS.

If you need to download specific point release you can visit http://cdimage.ubuntu.com/ubuntu-mate/releases/ .