Rest of the decimal units measured things that did not have any natural ratio. So there was nothing inherent about length that required 1ft=12in, so we might as well have all conversion ratios be a power of 10.

But with the calendar there is a natural ratio that is not a power of 10: 1year=365.2.. days. So using a decimal calendar means either giving up the concept of day by, say, dividing the year into 1000 units of ~8hrs or be content with having a non-decimal conversion ratio. In the calendar that was used in France for few years, 1 year had 12 months, each month had 3 decades, and each decade had 10 days. So conversion ratios between successively shorter time spans were 12, 3, and 10. Also, people had religious reasons for treating Sunday differently (which still came once every 7 days), and workers wouldn't have been too happy about rest 1 day in a decade rather than a week. This was not in any sense better than the existing Gregorian calendar.

The decimal clock was a nice idea. 1day = 10hr = 1,000 minutes = 100,000 seconds. I have no idea why it was not accepted more widely.