I haven't followed all the developments in the Biden administration suspension of the death penalty, but some of the concerns seem to be about methods rather than whether it should ultimately resumed at all:
US Attorney General Merrick Garland has ordered a moratorium on federal executions while the Justice Department continues its review of the death penalty, the department said in a statement. [...]
Garland said on Thursday [July 1] that he was directing Deputy Attorney General Lisa Monaco to lead a review that will assess, among other things, the Justice Department’s 2019 single-drug protocol as well as “the risk of pain and suffering associated with the use of pentobarbital”.
The review will also scrutinise changes the Justice Department made to its regulations in November 2020 that expanded the methods it could use to carry out executions.
The DOJ press release at the time also emphasized these reasons:
In the last two years, the department made a series of changes to capital case policies and procedures and carried out the first federal executions in nearly two decades between July 2020 and January 2021. That included adopting a new protocol for administering lethal injections at the federal Bureau of Prisons, using the drug pentobarbital. Attorney General Garland’s memorandum directs the Deputy Attorney General to lead a multi-pronged review of these recent policy changes, including:
A review coordinated by the Office of Legal Policy of the Addendum to the Federal Execution Protocol, adopted in 2019, which will assess, among other things, the risk of pain and suffering associated with the use of pentobarbital.
- A review coordinated by the Office of Legal Policy to consider changes to Justice Department regulations made in November 2020 that expanded the permissible methods of execution beyond lethal injection, and authorized the use of state facilities and personnel in federal executions.
- A review of the Justice Manual’s capital case provisions, including the December 2020 and January 2021 changes to expedite execution of capital sentences.
- The Attorney General’s memorandum requires the reviews to include consultations with a wide range of stakeholders including the relevant department components, other federal and state agencies, medical experts and experienced capital counsel, among others.
No federal executions will be scheduled while the reviews are pending.
It may have been just politically convenient to suspend it for this reason/pretext, and the Biden administration might not actually plan to resume it while in power, but I haven't found statements to that effect (yet).
Also, worth noting perhaps, the appeal on Tsarnaev's sentence was actually initiated by the DOJ while Trump was still president. As noted in the Hill on July 22, Biden's DOJ did move to no longer call for the death penalty in some 7 cases that were also initiated under Trump. So perhaps they consider Tsarnaev's case exceptional (for either legal or political reasons).
Biden himself said as a campaign promise he'd help pass a law to end the death penalty, but that could also be seen as a bit of a way to give something to the death-penalty abolitionists in his party
while probably knowing full well that such a law would be hard to pass due to the continued popularity of the death penalty with the US public, even if it's trending down. (A bill like that has died numerous times in committees.)
And if only look to what happened last year with the election, the DOJ doesn't always just go with the president's wishes...