The limiting factor on the maximum level of power an Ardent can learn is, as speculated in the question and correctly pointed out in Ben Barden's answer, the limit on the number of power points a manifester can spend on a single power. From XPH (and the SRD):
you can’t spend more power points on a power than your manifester level
Your manifester level is specific to each psionic class, just like caster level. From the SRD:
The variables of a power’s effect often depend on its manifester level, which is equal to your psionic class level.
However, it's important to note that manifester level is not the same thing as Ardent level. Your manifester level (and thereby the number of power points you can spend on a single power) can be increased by things like:
- The Practiced Manifester feat (CPsi), which provides +4 ML to a maximum of your total HD
- Arguably the Overchannel feat, though some DMs may rule that Overchannel applies only at the time of manifesting and therefore doesn't factor into the maximum level of power you can manifest
- Depending on your DM's rulings on Magic/Psionics transparency (and potentially also the Ardent's Magic mantle), anything that increases caster level, such as an orange ioun stone or a bead of karma
This means that Ardents (especially multiclass Ardents, who can get +4 ML, no questions asked, for the bargain price of a single feat) can potentially learn high-level powers with an investment of relatively few class levels. For instance, an Ardent 1/Barbarian 3 with the Practiced Manifester feat could take their second level of Ardent at level 5 (becoming an Ardent 2/Barbarian 3) and learn a third level power.
According to Nijineko in the comments, and corroborated in an old forum thread, the author of the class is apparently on record as saying there were intended to be stricter limits to the maximum level of Ardent powers known. As far as I can tell, though, this never made it into any official rules source like an errata document, or even a quasi-official source like an FAQ. I am unable to find the original comment at all; it may have been lost in one of the WotC forum purges.
The balance implications of this situation, and any authorially-approved houserules to correct it, are left as an exercise for the reader and/or a conversation to have with your DM.