Because HoTMs is most easier non-S1 5* to get, should they give more benefit to f2p/c2p than to p2w?
I think event heroes should be p2w powerhouses, niche heroes which can hardly be beaten by s1 heroes.
Iconic example of such hero are Guin.
HoTMs, in my opinion, should be more universal and usable almost everywhere, but not best of the best.
For me, good examples of HoTMs are Seshat, Delilah, GM and Clarissa.
Bad examples is Miki and Telluria, extremely good in their niche, but (for Miki), is not good in other places, or OP heroes like Telluria (but discussing Telluria is off-topic here).
More clear idea - S1 heroes should be all-round heroes, HoTMs more niche (slightly better than S1 in their niche, but on par in other niches), and event heroes should be way better on their niche than S1, slightly better than HOTM, but in other niches event heroes should be worse than S1.