In response to why people are lax with the issur chodosh nowadays, R. J. Waxman suggests 4 potential halachik arguments. Unfortunately, none of these reasons withstand real scrutiny:
The Rama cited is quoting a Rosh who explicitly requires investigating whether there is indeed a double uncertainty. If it can be reasonably determined that the grain is at least as likely chodosh as not, the Rama would forbid it, with the caveat that one shouldn’t inform those of the problem if they would not correct their practice in response (mutav sheyehu shoggegin v’al yehu mezidin). Considering that those who keep yoshon rely on a readily available "Guide to Chodosh" to determine cutoff dates based on USDA crop reports, the Rama’s leniency seems poorly applied to the issue of contemporary chodosh consumption.
Besides for the concern raised (e.g. by the Shach) that the leniency of “kdai hu lismoch alav b’shaas hadechak” is only applied in the gemara to Rabbinic prohibitions, never biblical ones, there is a fundamental problem with using the Taz’s kula. In most contemporary Jewish communities, it’s simply no longer a shaas hedechak in that comparably priced yoshon alternatives are readily available.
3-4. It’s also not so simple to rely on the chiddush of the Bach (or Knesses Yechezkel) since he is explicitly arguing with the mainstream rishonim and is generally rejected by the subsequent achronim, including the GR”A, Shach, and Taz (to name a few).
Because 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 kula nowadays is based on a "perfect storm" of factors that caused Jews otherwise quite strict in matters of kashrus to be lax with regard to chodosh.
Firstly, as opposed to other maachalos assuros which are fairly fixed, chodosh varies in applicability both by location and season. As such, communities that never had to worry about chodosh, 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.
In addition, I would add that nowadays people generally rely on popularly accepted kosher symbols which shifts the a question to why are the rabbis behind the symbols lax. With regard to the most famous kosher symbol, the O-U, Kashrus guidelines are set by Rav Herschel Schachter, Rav Yisrael Belsky, and Rav Menachem Genack.
Rav Schachter justifies the custom based on the Aruch Hashulchan who cites the then recently discovered Ohr Zarua that was melamed zechus by combining 2 chidushim: 1. We follow the view that Chodosh in chutz laaretz is only miderabanan; 2. that gezeira was only applied to nearby lands to Israel. Rav Belsky has suggested that while Hassidim perhaps rely on the Bach, misnagdim should really follow the Rama who forbids chodosh. He subsequently suggested that even misnagdim perhaps rely on the Bach. Rav Belsky himself, as well as Rav Genack, are themselves strict and don't eat chodosh (though the O-U does still certify it, even apparently where there is no doubt that it is chodosh. - perhaps they reason that for the O-U it would be a shaas hadechak since they would lose clients;).
(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.