Because Microsoft doesn't release it to the entire world at the same time.  They stagger the rollout, and initially even the "seekers" (the ones who check for updates manually) don't all get access to it at the same time.  If you really want it now though, you can download it here.  Note that due to a known issue with the update as of this writing, you will have to disconnect any USB storage (hard drives and flash drives) and any memory cards during the update.  They'll work fine afterward, but apparently there's some issue where drive letters could get reassigned, which would break the update process, so Microsoft implemented this restriction for installing the update.  In fact if you currently have a device like that attached, that could also be why you're not seeing the update.  Windows Update might be determining that your PC isn't "eligible" to install it at the moment.  Either way, the manual updater above will get you to 1903.

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@jphughan wrote:

Because Microsoft doesn't release it to the entire world at the same time.  They stagger the rollout, and initially even the "seekers" (the ones who check for updates manually) don't all get access to it at the same time.  If you really want it now though, you can download it here.  Note that due to a known issue with the update as of this writing, you will have to disconnect any USB storage (hard drives and flash drives) and any memory cards during the update.  They'll work fine afterward, but apparently there's some issue where drive letters could get reassigned, which would break the update process, so Microsoft implemented this restriction for installing the update.  In fact if you currently have a device like that attached, that could also be why you're not seeing the update.  Windows Update might be determining that your PC isn't "eligible" to install it at the moment.  Either way, the manual updater above will get you to 1903.

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the problem with USB storage in 1903 has been fixed with the KB4497935 update (build 18362.145), however that update is currently available to insiders and that update may get officially released sometime next week.

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Addresses an issue that may cause an external USB device or SD memory card to be reassigned to an incorrect drive during installation.

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I find that it is best to wait a few more weeks until the 1903 upgrade is delivered thru Windows Update which will obtain the newest build & that it contains the usb storage fix.

I had good luck using a 32 GB flash drive to install.  The smaller ones would not even allow me to get to the actual install part before a message about not being able to update showed up.

I do not know why since the data on the drive is only around 4 GB, but it happened on four different installs.

For info Purposes:

https://answers.microsoft.com/en-us/windows/forum/all/may-1903-update-is-out/2aa24f4b-caa6-4a62-8dd9-f3123fc13803

Perhaps wait another month or so to install.

You can download the Microsoft Media creation tool for Windows 10 1903. When you run it the app will ask you do you want to upgrade this PC or create a install image for another. It will then proceed to install the upgrade if your device doesn't have any blocks preventing it from upgrading. My personally advice is don't be so eager to upgrade as you may experience problems. I would wait until your device notifies you of the upgrade available for your PC. 

To download and install the latest version 1803 using Windows Update, the steps:
Open Settings. Click on Update & Security.
Click the Check for Updates button.
After the update finishes in the download, click the "Restart Now" button

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@roy1991 wrote:

To download and install the latest version 1803 using Windows Update, the steps:
Open Settings. Click on Update & Security.
Click the Check for Updates button.
After the update finishes in the download, click the "Restart Now" button

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As mentioned in the first post of this thread, the OP already did that and it wasn’t being offered that way.

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@HaseebSE wrote:

Hi everyone. My laptop is dell Inspiron 5548. My window 10 version is 1809. My update service is good.  I rechecking every 1 hours  But. Not getting. May 2019 update. Why

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I also have a Dell Inspiron 5548 that won't update to 1903 when using Windows Update.  I thought the reason I wasn't getting the update was that the 5548 wasn't tested for 1903.  I never got the update for 1809 either.

Dell computers tested for Windows 10

So, it's safe for me to force the update?

Did you read the previous comments made on this thread??

If you must have now:   https://www.microsoft.com/en-us/software-download/windows10

I suggest waiting 2-3 months until they have most ALL bugs worked out.

You can go here for more info if you have a Microsoft account and sign into their Community for more info:

https://answers.microsoft.com/en-us

Hope this helps and good luck