Because Allison looks for more in a partner than "motivation" and "ambition". She's had a crush on Ian for a long time, what she sees in him is much more than initial attraction. Allison is somebody who simply wants to be appreciated, she's pretty insecure as much as she acts like she isn't, and wants some sense of validation. Ian gives her all that in a genuine way, he's attentive, supportive, and actually really likes her as an individual(so his compliments are genuine), add in that they've known each other for a long time, which means he's knows her really well, and you have exactly what she wants. Somebody who sees her for who she is, and loves that. Her opinion on Ian's "ambitiousness" seems to be pretty positive as well, he's actully taking steps towards his goals, even if it seems lofty. Honeslty though as much as she seems like somebody who has "narrow standards", her expectations for a partner are actually pretty fair.