"Why is it some suppliers can, and others can't?!"
Because some can, and some cannot. Even some of the 'big names' have had problems with deliveries and supplies, if media reports are anything to note.
Over the past couple of weeks, I have received next day supplies, and other deliveries that were promised took a little longer. I have a number of items coming from China and South East Asia, that in the main were not promised till the New Year, yet have started to arrive now (2 weeks from order).
What is becoming more noticeable with previously non-EU overseas goods, is how some sellers are using EU/UK distribution facilities more for UK deliveries. This not only provides a better and faster service, but cut-back on any paperwork or questions on possible extra costs.
One thing that I have noticed, is the Royal Mail 24 or 48 hour service, which seems to be taking much longer nowadays. Royal Mail are still complaining that they are losing profits to competitor's, yet in the main, all our small mail deliveries are via Royal Mail post-person's hand deliveries. Just doesn't make sense!.