Penelope made a promise to Odysseus about time. Telemachus was born just before Odysseus left. Odysseus encouraged Penelope that an appropriate time to trust that Odysseus might not be returning at all would be when Telemachus face begin to bear facial hair. At that point, she must look for a suitable replacement. These men are aware that Telemachus' time is coming soon for puberty has well begun in him.
A few other logical features exist to her problem. There are more of them that there are of her. She was also working on a shroud for Laertes. She promised these men that as soon as the shroud was complete, she would indeed choose a suitor. The clock is ticking and she is Ithaca's most eligible bachelorette.