It's very probable that Shaddam can simply no longer afford the insanely enormous amount of spice drug that is keeping him looking younger than he is.
Not only is Shaddam no longer Emperor, he is no longer obscenely wealthy. He lost almost everything when he was deposed. And it's worth remembering that he also spent a mega fortune on that plan to send that enormous legion of Sardaukar to Arrakis to aid the Harkonnens in destroying their mutual enemy. That was an enormous investment that was expected to have a giant payoff.
During the Jihad, Paul's warriors likely confiscated Shaddam's secret stash of hoarded spice, just like they did with the Harkonnens when taking over Geidi Prime. Shaddam and his family were probably not allowed so much as to return to Kaitain and pack up their things before going to their new home on Salusa Secundis.
Most likely, Shaddam, along with maintaining a comfortable lifestyle for his family on S. Secundis, has to use his remaining wealth (which is probably now down there with the pre-ascension Atreides) to recruit/bribe supporters in an attempt to make a move to take back the throne. He just has had to budget himself in the years after his depose. Likely, he has to make less spice go a long way but when using the spice to extend life and keep oneself younger looking than they are, it becomes spice addiction and the dosage increases incredibly. It is reasonably that Shaddam, in having to make do with less, eventually succumbed to the addiction withdrawal.
Emperor Paul has no reason to just give spice to Shaddam even though it would be barely a drop in the bucket for him. And Shaddam knew to expect no charity. Shaddam was no tired old man who just wanted to live out his remaining years in comfort. He was still potentially dangerous and would have probably influenced Wensicia behind the scenes had he lived.