Josh Waxman suggests 4 reasonsIn response to why people are lax with the issur chodoshissur chodosh nowadays, R. J. Waxman suggests 4 potential halachik arguments. Unfortunately, none of themthese reasons withstand real scrutiny:
InsteadBecause of these weaknesses, and for the sake of intellectual honesty, I would argue that the basis for the lax custom is better answered from a socio-historical perspective rather than a strict Halachik one. Specifically, I would suggest that the kulakula nowadays is based on a "perfect storm" of factors that caused Jews who otherwise are if anything excessivelyquite strict (at least in ritual domains)matters of kashrus to be lax in this particular onewith regard to chodosh.
Firstly, as opposed to other maachalos assuros which are fairly fixed, chodoshchodosh varies in applicability both by location and season. As such, communities that never had to worry about chodoshchodosh, when transplanted to a different locale, suddenly did. Furthermore, since alternatives were not readily available, there was a genuine concern on the part of poskim that a stringent ruling would just be ignored (see Rama and Rosh mentioned earlier "mutav sheyehu shoggegin"). In addition, there is a fairly obvious trend in the derech hapesak of Chachmei Ashkenaz going back to the Baalei Tosfos to be "melamed zechus" on a minhag yisrael. As such a diverse set of arguments were presented to try and justify the lenient attitude. (Hassidim, who often are amongst the most stringent on issues of maachalos assuros, are also lax because of a tradition that the Bach appeared in a dream to the Besht and encouraged him to be lax. Interestingly, it's recorded that while the Besht was indeed lenient for a while, he subsequently returned to his original observing of yoshon.) Finally, in an ironic twist on "chodosh assur min hatorah" there is a contemporary popular view in right-wing circles to keep old practices even when the original reasoning no longer applies. In spite of this, there is a growing trend toward yoshon-observance.
(Rav Mordechai Willig also rules leniently, though he apparently doesn't trust the USDA crop reports or the guide to chodosh and maintains that US grain doesn't hit store shelves till it's yoshon. This reasoning may also be used by Rav Nachum Rabinovitch who I believe also invokes the Taz in his sefer, Siach Nachum. The O-U presumably does not cite such a position since they themselves separately certify yoshon flour for bakeries and restaurants which they discontinue once the flour is chodosh [at which point they just affix a regular O-U].)
Finally, with regard to the final question of: "Is it a commandment..." The answer is yes. No posek matirs chodosh in Eretz Yisrael since that is not up for debate. It has been a generational practice however to justify the already prevalent lax attitude in the Diaspora where there is more room for debate, at least with the d'oraissa issue. I think every intellectually honest person would agree that none of the lax positions would have been taken, however, had the masses not already been lax, or (at least in earlier generations) had they been likely to follow a ban on the practice.