Why is Higgs particle detected much later than top quark when it's lighter?

With both particles you cannot detect them within their own lifetimes, only look at what they decay into. The top decays to a b jet and W (which can then become fermion anti-fermion or leptons) and is fairly distinctive. The dominant Higgs decay, however, is to two b jets. B jets are very common within the LHC and we cannot infer from two b jets that a Higgs detection has been made.

It's all about the statistics. With the top, its decay mode was distinctive enough that fewer events were needed before a statistically significant signal was seen above the background, whereas the Higgs needed many more events.