When it was clear Conte was going to Chelsea, they were still reigning champions. He made one key signing in Kante (and a couple of others but less key imo), and now they are champions again, on course for a 90 point season. Hazard and Costa weren't in their final year and basically gone when he arrived either.
Mourinho was unemployed after already working at many of Europe's top teams who wouldn't have him back and he went to the biggest club in the country and was given a 150m budget. He'd also wanted that job for a loooong time.
Not saying we won't get someone good but someone might think twice if they have another offer at the same time. Whatever way you look at it, take over a club in 2nd/3rd and your job is to get them 1st/2nd. Take over a club in 6th, your job is to get 4th and that's a different proposition altogether.