This is a common misconception about HotM. There isn’t a “finite pool” of HotM which gets depleted over time.
Every summons has an equal chance of giving the bonus draw. Even the bonus draw has a chance for having a bonus draw
What happened with Telluria is that every man & his dog decided to save gems or spend heaps of gems summoning for Telluria. Then pretty much everyone who got her dumped all their other projects and levelled her up to put on their defence teams…
Reason being is that she was known to be & instantly apparent to be a very powerful hero (am not stating OP or not, just the fact that she is a very powerful hero).